[PowerPC] Implement Move to VSR Mask builtins in LLVM/Clang

[PowerPC] Implement Move to VSR Mask builtins in LLVM/Clang

This patch implements the vec_gen[b|h|w|d|q]m function prototypes in altivec.h
in order to utilize the move to VSR with mask instructions introduced in Power10.

Differential Revision: https://reviews.llvm.org/D82725