x86-64: Optimize strcat/strncat, strcpy/strncpy and stpcpy/stpncpy with AVX2

x86-64: Optimize strcat/strncat, strcpy/strncpy and stpcpy/stpncpy with AVX2

Optimize x86-64 strcat/strncat, strcpy/strncpy and stpcpy/stpncpy with AVX2.
It uses vector comparison as much as possible. In general, the larger the
source string, the greater performance gain observed, reaching speedups of
1.6x compared to SSE2 unaligned routines. Select AVX2 strcat/strncat,
strcpy/strncpy and stpcpy/stpncpy on AVX2 machines where vzeroupper is
preferred and AVX unaligned load is fast.

	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	strcat-avx2, strncat-avx2, strcpy-avx2, strncpy-avx2,
	stpcpy-avx2 and stpncpy-avx2.
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c:
	(__libc_ifunc_impl_list): Add tests for __strcat_avx2,
	__strncat_avx2, __strcpy_avx2, __strncpy_avx2, __stpcpy_avx2
	and __stpncpy_avx2.
	* sysdeps/x86_64/multiarch/{ifunc-unaligned-ssse3.h =>
	ifunc-strcpy.h}: rename header for a more generic name.
	* sysdeps/x86_64/multiarch/ifunc-strcpy.h:
	(IFUNC_SELECTOR): Return OPTIMIZE (avx2) on AVX 2 machines if
	AVX unaligned load is fast and vzeroupper is preferred.
	* sysdeps/x86_64/multiarch/stpcpy-avx2.S: New file
	* sysdeps/x86_64/multiarch/stpncpy-avx2.S: Likewise
	* sysdeps/x86_64/multiarch/strcat-avx2.S: Likewise
	* sysdeps/x86_64/multiarch/strcpy-avx2.S: Likewise
	* sysdeps/x86_64/multiarch/strncat-avx2.S: Likewise
	* sysdeps/x86_64/multiarch/strncpy-avx2.S: Likewise