x86-64: Optimize memcmp/wmemcmp with AVX2 and MOVBE

x86-64: Optimize memcmp/wmemcmp with AVX2 and MOVBE

Optimize x86-64 memcmp/wmemcmp with AVX2.  It uses vector compare as
much as possible.  It is as fast as SSE4 memcmp for size <= 16 bytes
and up to 2X faster for size > 16 bytes on Haswell and Skylake.  Select
AVX2 memcmp/wmemcmp on AVX2 machines where vzeroupper is preferred and
AVX unaligned load is fast.

NB: It uses TZCNT instead of BSF since TZCNT produces the same result
as BSF for non-zero input.  TZCNT is faster than BSF and is executed
as BSF if machine doesn't support TZCNT.

Key features:

1. For size from 2 to 7 bytes, load as big endian with movbe and bswap
   to avoid branches.
2. Use overlapping compare to avoid branch.
3. Use vector compare when size >= 4 bytes for memcmp or size >= 8
   bytes for wmemcmp.
4. If size is 8 * VEC_SIZE or less, unroll the loop.
5. Compare 4 * VEC_SIZE at a time with the aligned first memory area.
6. Use 2 vector compares when size is 2 * VEC_SIZE or less.
7. Use 4 vector compares when size is 4 * VEC_SIZE or less.
8. Use 8 vector compares when size is 8 * VEC_SIZE or less.

	* sysdeps/x86/cpu-features.h (index_cpu_MOVBE): New.
	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	memcmp-avx2 and wmemcmp-avx2.
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Test __memcmp_avx2 and __wmemcmp_avx2.
	* sysdeps/x86_64/multiarch/memcmp-avx2.S: New file.
	* sysdeps/x86_64/multiarch/wmemcmp-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/memcmp.S: Use __memcmp_avx2 on AVX
	2 machines if AVX unaligned load is fast and vzeroupper is
	preferred.
	* sysdeps/x86_64/multiarch/wmemcmp.S: Use __wmemcmp_avx2 on AVX
	2 machines if AVX unaligned load is fast and vzeroupper is
	preferred.