[PowerPC] Provide MMA builtins for compatibility

Vector pair intrinsics and builtins were renamed in
https://reviews.llvm.org/D91974 to replace the _mma_ prefix by _vsx_.
However, some projects used the _mma_ version, so this patch adds
these intrinsics to provide compatibility.

Fixes Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=50159

Reviewed By: nemanjai, amyk

Differential Revision: https://reviews.llvm.org/D100482