x86-64: Skip zero length in __mem[pcpy|move|set]_erms

This patch skips zero length in __mempcpy_erms, __memmove_erms and
__memset_erms.

Tested on x86-64.

	* sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
	(__mempcpy_erms): Skip zero length.
	(__memmove_erms): Likewise.
	* sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
	(__memset_erms): Likewise.