[AArch64] Optimized memcmp.

This is an optimized memcmp for AArch64.  This is a complete rewrite
using a different algorithm.  The previous version split into cases
where both inputs were aligned, the inputs were mutually aligned and
unaligned using a byte loop.  The new version combines all these cases,
while small inputs of less than 8 bytes are handled separately.

This allows the main code to be sped up using unaligned loads since
there are now at least 8 bytes to be compared.  After the first 8 bytes,
align the first input.  This ensures each iteration does at most one
unaligned access and mutually aligned inputs behave as aligned.
After the main loop, process the last 8 bytes using unaligned accesses.

This improves performance of (mutually) aligned cases by 25% and
unaligned by >500% (yes >6 times faster) on large inputs.

	* sysdeps/aarch64/memcmp.S (memcmp):
	Rewrite of optimized memcmp.

(cherry picked from commit 92236903)