[PPC][GlobalISel] Add initial GlobalIsel infrastructure

This adds the initial GlobalISel skeleton for PowerPC. It can only run
ir-translator and legalizer for `ret void`.

This is largely based on the initial GlobalISel patch for RISCV
(https://reviews.llvm.org/D65219).

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D83100