aarch64: Add memmove expansion for +mops

aarch64: Add memmove expansion for +mops

This second patch in the series adds an inline movmem expansion for TARGET_MOPS
that emits the recommended sequence.

A new param aarch64-mops-memmove-size-threshold is added to control the memmove size threshold
for this expansion. Its default value is zero to be consistent with the current behaviour where
we always emit a libcall, as we don't currently have a movmem inline expansion
(we should add a compatible-everywhere inline expansion, but that's for the future), so we should
always prefer to emit the MOPS sequence when available in lieu of a libcall.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/aarch64.md (aarch64_movmemdi): Define.
	(movmemdi): Define.
	(unspec): Add UNSPEC_MOVMEM.
	* config/aarch64/aarch64.opt (aarch64-mops-memmove-size-threshold):
	New param.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/mops_2.c: New test.