AArch64: Add support for MOPS memcpy/memmove/memset

Add support for MOPS in cpu_features and INIT_ARCH.  Add ifuncs using MOPS for
memcpy, memmove and memset (use .inst for now so it works with all binutils
versions without needing complex configure and conditional compilation).

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>