[AIX] Change the default target CPU to power4 for AIX on Power

Summary: This patch changes the AIX default target CPU to power4 since this is the the lowest arch for the lowest OS level supported.

Reviewers: hubert.reinterpretcast, cebowleratibm, daltenty

Reviewed By: hubert.reinterpretcast

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80835