aarch64: Improve strcmp unaligned performance

Replace the simple byte-wise compare in the misaligned case with a
dword compare with page boundary checks in place.  For simplicity I've
chosen a 4K page boundary so that we don't have to query the actual
page size on the system.

This results in up to 3x improvement in performance in the unaligned
case on falkor and about 2.5x improvement on mustang as measured using
bench-strcmp.

	* sysdeps/aarch64/strcmp.S (misaligned8): Compare dword at a
	time whenever possible.

(cherry picked from commit 2bce01eb)