[PowerPC][AIX] Make PIC the default relocation model for AIX

Summary:
The `llc` tool currently defaults to Static relocation model and generates non-relocatable code for 32-bit Power.
This is not desirable on AIX where we always generate Position Independent Code (PIC). This patch makes PIC the default relocation model for AIX.

Reviewers: daltenty, hubert.reinterpretcast, DiggerLin, Xiangling_L, sfertile

Reviewed By: hubert.reinterpretcast

Subscribers: mgorny, wuzish, nemanjai, hiraditya, kbarton, jsji, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72479