x86-64: Optimize memrchr with AVX2

Optimize memrchr with AVX2 to search 32 bytes with a single vector
compare instruction.  It is as fast as SSE2 memrchr for small data
sizes and up to 1X faster for large data sizes on Haswell.  Select
AVX2 memrchr on AVX2 machines where vzeroupper is preferred and AVX
unaligned load is fast.

	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	memrchr-sse2 and memrchr-avx2.
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Add tests for __memrchr_avx2 and
	__memrchr_sse2.
	* sysdeps/x86_64/multiarch/memrchr-avx2.S: New file.
	* sysdeps/x86_64/multiarch/memrchr-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/memrchr.c: Likewise.