aarch64: Use +mops to inline memset operations

aarch64: Use +mops to inline memset operations

This 3rd patch in the series adds an inline sequence for the memset operation.
The aarch64-mops-memset-size-threshold param is added to control the size threshold for the sequence.
Its default setting is 256, which may seem a bit high, but it is consistent with the current
SIMD memset inline sequence limit, and future CPU tunings can override it easily as needed.

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

gcc/ChangeLog:

	* config/aarch64/aarch64.c (aarch64_expand_setmem_mops): Define.
	(aarch64_expand_setmem): Adjust for TARGET_MOPS.
	* config/aarch64/aarch64.h (CLEAR_RATIO): Adjust for TARGET_MOPS.
	(SET_RATIO): Likewise.
	* config/aarch64/aarch64.md ("unspec"): Add UNSPEC_SETMEM.
	(aarch64_setmemdi): Define.
	(setmemdi): Adjust for TARGET_MOPS.
	* config/aarch64/aarch64.opt (aarch64-mops-memset-size-threshold):
	New param.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/mops_3.c: New test.