Add x86-64 memset with unaligned store and rep stosb

Implement x86-64 memset with unaligned store and rep movsb.  Support
16-byte, 32-byte and 64-byte vector register sizes.  A single file
provides 2 implementations of memset, one with rep stosb and the other
without rep stosb.  They share the same codes when size is between 2
times of vector register size and REP_STOSB_THRESHOLD which defaults
to 2KB.

Key features:

1. Use overlapping store to avoid branch.
2. For size <= 4 times of vector register size, fully unroll the loop.
3. For size > 4 times of vector register size, store 4 times of vector
register size at a time.

	[BZ #19881]
	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	memset-sse2-unaligned-erms, memset-avx2-unaligned-erms and
	memset-avx512-unaligned-erms.
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Test __memset_chk_sse2_unaligned,
	__memset_chk_sse2_unaligned_erms, __memset_chk_avx2_unaligned,
	__memset_chk_avx2_unaligned_erms, __memset_chk_avx512_unaligned,
	__memset_chk_avx512_unaligned_erms, __memset_sse2_unaligned,
	__memset_sse2_unaligned_erms, __memset_erms,
	__memset_avx2_unaligned, __memset_avx2_unaligned_erms,
	__memset_avx512_unaligned_erms and __memset_avx512_unaligned.
	* sysdeps/x86_64/multiarch/memset-avx2-unaligned-erms.S: New
	file.
	* sysdeps/x86_64/multiarch/memset-avx512-unaligned-erms.S:
	Likewise.
	* sysdeps/x86_64/multiarch/memset-sse2-unaligned-erms.S:
	Likewise.
	* sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S:
	Likewise.

(cherry picked from commit 83056630)