X86-64: Prepare memset-vec-unaligned-erms.S

Prepare memset-vec-unaligned-erms.S to make the SSE2 version as the
default memset.

	* sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
	(MEMSET_CHK_SYMBOL): New.  Define if not defined.
	(__bzero): Check VEC_SIZE == 16 instead of USE_MULTIARCH.
	Disabled fro now.
	Replace MEMSET_SYMBOL with MEMSET_CHK_SYMBOL on __memset_chk
	symbols.  Properly check USE_MULTIARCH on __memset symbols.