x86-64: Optimize strlen/strnlen/wcslen/wcsnlen with AVX2

Optimize strlen/strnlen/wcslen/wcsnlen with AVX2 to check 32 bytes with
a single vector compare instruction.  It is as fast as SSE2 versions for
size <= 16 bytes and up to 1X faster for or size > 16 bytes on Haswell.
Select AVX2 version 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/multiarch/Makefile (sysdep_routines): Add
	strlen-sse2, strnlen-sse2, strlen-avx2, strnlen-avx2,
	wcslen-sse2, wcslen-avx2 and wcsnlen-avx2.
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Add tests for __strlen_avx2,
	__strlen_sse2, __strnlen_avx2, __strnlen_sse2, __wcslen_avx2,
	__wcslen_sse2 and __wcsnlen_avx2.
	* sysdeps/x86_64/multiarch/strlen-avx2.S: New file.
	* sysdeps/x86_64/multiarch/strlen-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/strlen.c: Likewise.
	* sysdeps/x86_64/multiarch/strnlen-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/strnlen-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/strnlen.c: Likewise.
	* sysdeps/x86_64/multiarch/wcslen-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/wcslen-sse2.S: Likewise.
	* sysdeps/x86_64/multiarch/wcslen.c: Likewise.
	* sysdeps/x86_64/multiarch/wcsnlen-avx2.S: Likewise.
	* sysdeps/x86_64/multiarch/wcsnlen.c (OPTIMIZE (avx2)): New.
	(IFUNC_SELECTOR): Return OPTIMIZE (avx2) on AVX2 machines where
	vzeroupper is preferred and AVX unaligned load is fast.