aarch64: Improve strncmp for mutually misaligned inputs

The mutually misaligned inputs on aarch64 are compared with a simple
byte copy, which is not very efficient.  Enhance the comparison
similar to strcmp by loading a double-word at a time.  The peak
performance improvement (i.e. 4k maxlen comparisons) due to this on
the strncmp microbenchmark is as follows:

falkor: 3.5x (up to 72% time reduction)
cortex-a73: 3.5x (up to 71% time reduction)
cortex-a53: 3.5x (up to 71% time reduction)

All mutually misaligned inputs from 16 bytes maxlen onwards show
upwards of 15% improvement and there is no measurable effect on the
performance of aligned/mutually aligned inputs.

	* sysdeps/aarch64/strncmp.S (count): New macro.
	(strncmp): Store misaligned length in SRC1 in COUNT.
	(mutual_align): Adjust.
	(misaligned8): Load dword at a time when it is safe.

(cherry picked from commit 7108f1f9)