aarch64: Optimized memcmp for medium to large sizes

This improved memcmp provides a fast path for compares up to 16 bytes
and then compares 16 bytes at a time, thus optimizing loads from both
sources.  The glibc memcmp microbenchmark retains performance (with an
error of ~1ns) for smaller compare sizes and reduces up to 31% of
execution time for compares up to 4K on the APM Mustang.  On Qualcomm
Falkor this improves to almost 48%, i.e. it is almost 2x improvement
for sizes of 2K and above.

	* sysdeps/aarch64/memcmp.S: Widen comparison to 16 bytes at a
	time.

(cherry picked from commit 30a81dae)