x86_64: Remove redundant REX bytes from memchr.S

By x86-64 specification, 32-bit destination registers are zero-extended
to 64 bits.  There is no need to use 64-bit registers when only the lower
32 bits are non-zero.

	* sysdeps/x86_64/memchr.S (MEMCHR): Use 32-bit registers for
	the lower 32 bits.