x86_64: Remove redundant REX bytes from memrchr.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.  Also 2 instructions in:

	mov	%rdi, %rcx
	and	$15, %rcx
	jz	L(length_less16_offset0)

	mov	%rdi, %rcx		<<< redundant
	and	$15, %rcx		<<< redundant

are redundant.

	* sysdeps/x86_64/memrchr.S (__memrchr): Use 32-bit registers for
	the lower 32 bits.  Remove redundant instructions.