x86-64: Optimize memchr/rawmemchr/wmemchr with SSE2/AVX2

SSE2 memchr is extended to support wmemchr.  AVX2 memchr/rawmemchr/wmemchr
are added to search 32 bytes with a single vector compare instruction.
AVX2 memchr/rawmemchr/wmemchr are as fast as SSE2 memchr/rawmemchr/wmemchr
for small sizes and up to 1.5X faster for larger sizes on Haswell and
Skylake.  Select AVX2 memchr/rawmemchr/wmemchr 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.

	* sysdeps/x86_64/memchr.S (MEMCHR): New.  Depending on if
	USE_AS_WMEMCHR is defined.
	(PCMPEQ): Likewise.
	(memchr): Renamed to ...
	(MEMCHR): This.  Support wmemchr if USE_AS_WMEMCHR is defined.
	Replace pcmpeqb with PCMPEQ.
	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	memchr-sse2, rawmemchr-sse2, memchr-avx2, rawmemchr-avx2,
	wmemchr-sse4_1, wmemchr-avx2 and wmemchr-c.
	* sysdeps/x86_64/multiarch/ifunc-avx2.h: New file.
	* sysdeps/x86_64/multiarch/memchr-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/memchr-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/memchr.c: Likewise.
	* sysdeps/x86_64/multiarch/rawmemchr-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/rawmemchr-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/rawmemchr.c: Likewise.
	* sysdeps/x86_64/multiarch/wmemchr-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/wmemchr-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/wmemchr.c: Likewise.
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Test __memchr_avx2, __memchr_sse2,
	__rawmemchr_avx2, __rawmemchr_sse2, __wmemchr_avx2 and
	__wmemchr_sse2.

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