[AArch64] ACLE feature macro for Armv8.8-A MOPS

This introduces the new __ARM_FEATURE_MOPS ACLE feature test macro,
which signals the availability of the new Armv8.8-A/Armv9.3-A
instructions for standardising memcpy, memset and memmove operations.

This patch supersedes the one from https://reviews.llvm.org/D116160.

Differential Revision: https://reviews.llvm.org/D118199