This patch splits x86_64 out of the main Linux kernel-features.h.
Tested x86_64 that there are no changes to disassembly of installed
shared libraries.
* sysdeps/unix/sysv/linux/x86_64/kernel-features.h: New file.
* sysdeps/unix/sysv/linux/kernel-features.h [__x86_64__]
(__ASSUME_ACCEPT4_SYSCALL): Do not define.
[__LINUX_KERNEL_VERSION >= 0x020621 && __x86_64__]
(__ASSUME_RECVMMSG_SYSCALL): Likewise.
[__LINUX_KERNEL_VERSION >= 0x030000 && __x86_64__]
(__ASSUME_SENDMMSG_SYSCALL): Likewise.
[__x86_64__ && __LINUX_KERNEL_VERSION >= 0x030100]
(__ASSUME_GETCPU_SYSCALL): Likewise.
