Add MMA builtin decoding. These builtins use the new PowerPC-specific types __vector_pair and __vector_quad.
So to avoid pervasive changes, we use custom type descriptors and custom decoding of these builtins.
We also use custom code generation to expand builtin calls with pointers to simpler intrinsic calls with non-pointer types only.
This patch is part of the PowerPC MMA RFC that will be posted soon.