[AArch64] clang support for Armv8.8/9.3 MOPS

This introduces clang command line support for the new Armv8.8-A and
Armv9.3-A instructions for standardising memcpy, memset and memmove
operations, which was previously introduced into LLVM in
https://reviews.llvm.org/D116157.

Patch by Lucas Prates, Tomas Matheson and Son Tuan Vu.

Differential Revision: https://reviews.llvm.org/D117271