Как у тебя получилось? У меня сыпет ошибками:

/usr/include/linux/swab.h:136:23: error: expected ';' before 'unsigned'
  136 | static __always_inline unsigned long __swab(const unsigned long y)
      |                       ^~~~~~~~~
      |                       ;
  CC       /home/cool/linux-5.12/tools/objtool/elf.o
/usr/include/linux/swab.h:171:8: error: unknown type name '__always_inline'
  171 | static __always_inline __u16 __swab16p(const __u16 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/swab.h:171:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__swab16p'
  171 | static __always_inline __u16 __swab16p(const __u16 *p)
      |                              ^~~~~~~~~
/usr/include/linux/swab.h:184:8: error: unknown type name '__always_inline'
  184 | static __always_inline __u32 __swab32p(const __u32 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/swab.h:184:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__swab32p'
  184 | static __always_inline __u32 __swab32p(const __u32 *p)
      |                              ^~~~~~~~~
/usr/include/linux/swab.h:197:8: error: unknown type name '__always_inline'
  197 | static __always_inline __u64 __swab64p(const __u64 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/swab.h:197:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__swab64p'
  197 | static __always_inline __u64 __swab64p(const __u64 *p)
      |                              ^~~~~~~~~
/usr/include/linux/swab.h:252:23: error: expected ';' before 'void'
  252 | static __always_inline void __swab32s(__u32 *p)
      |                       ^~~~~
      |                       ;
/usr/include/linux/swab.h:265:23: error: expected ';' before 'void'
  265 | static __always_inline void __swab64s(__u64 *p)
      |                       ^~~~~
      |                       ;
In file included from /usr/include/asm/byteorder.h:5,
                 from /home/cool/linux-5.12/tools/arch/x86/include/asm/insn.h:10,
                 from arch/x86/decode.c:10:
/usr/include/linux/byteorder/little_endian.h:44:8: error: unknown type name '__always_inline'
   44 | static __always_inline __le64 __cpu_to_le64p(const __u64 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:44:31: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__cpu_to_le64p'
   44 | static __always_inline __le64 __cpu_to_le64p(const __u64 *p)
      |                               ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:48:8: error: unknown type name '__always_inline'
   48 | static __always_inline __u64 __le64_to_cpup(const __le64 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:48:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__le64_to_cpup'
   48 | static __always_inline __u64 __le64_to_cpup(const __le64 *p)
      |                              ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:52:8: error: unknown type name '__always_inline'
   52 | static __always_inline __le32 __cpu_to_le32p(const __u32 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:52:31: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__cpu_to_le32p'
   52 | static __always_inline __le32 __cpu_to_le32p(const __u32 *p)
      |                               ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:56:8: error: unknown type name '__always_inline'
   56 | static __always_inline __u32 __le32_to_cpup(const __le32 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:56:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__le32_to_cpup'
   56 | static __always_inline __u32 __le32_to_cpup(const __le32 *p)
      |                              ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:60:8: error: unknown type name '__always_inline'
   60 | static __always_inline __le16 __cpu_to_le16p(const __u16 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:60:31: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__cpu_to_le16p'
   60 | static __always_inline __le16 __cpu_to_le16p(const __u16 *p)
      |                               ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:64:8: error: unknown type name '__always_inline'
   64 | static __always_inline __u16 __le16_to_cpup(const __le16 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:64:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__le16_to_cpup'
   64 | static __always_inline __u16 __le16_to_cpup(const __le16 *p)
      |                              ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:68:8: error: unknown type name '__always_inline'
   68 | static __always_inline __be64 __cpu_to_be64p(const __u64 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:68:31: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__cpu_to_be64p'
   68 | static __always_inline __be64 __cpu_to_be64p(const __u64 *p)
      |                               ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:72:8: error: unknown type name '__always_inline'
   72 | static __always_inline __u64 __be64_to_cpup(const __be64 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:72:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__be64_to_cpup'
   72 | static __always_inline __u64 __be64_to_cpup(const __be64 *p)
      |                              ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:76:8: error: unknown type name '__always_inline'
   76 | static __always_inline __be32 __cpu_to_be32p(const __u32 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:76:31: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__cpu_to_be32p'
   76 | static __always_inline __be32 __cpu_to_be32p(const __u32 *p)
      |                               ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:80:8: error: unknown type name '__always_inline'
   80 | static __always_inline __u32 __be32_to_cpup(const __be32 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:80:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__be32_to_cpup'
   80 | static __always_inline __u32 __be32_to_cpup(const __be32 *p)
      |                              ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:84:8: error: unknown type name '__always_inline'
   84 | static __always_inline __be16 __cpu_to_be16p(const __u16 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:84:31: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__cpu_to_be16p'
   84 | static __always_inline __be16 __cpu_to_be16p(const __u16 *p)
      |                               ^~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:88:8: error: unknown type name '__always_inline'
   88 | static __always_inline __u16 __be16_to_cpup(const __be16 *p)
      |        ^~~~~~~~~~~~~~~
/usr/include/linux/byteorder/little_endian.h:88:30: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__be16_to_cpup'
   88 | static __always_inline __u16 __be16_to_cpup(const __be16 *p)
      |                              ^~~~~~~~~~~~~~

Ядро 5.11.16 конечно в Void собирается.