[PowerPC][AIX] Adds support for writing the data section in object files

Adds support for generating the XCOFF data section in object files for global variables with initialization.

Merged aix-xcoff-common.ll into aix-xcoff-data.ll.

Changed variable name charr to chrarray in the test case to test if readobj works with 8-character names.

Authored by: xingxue

Reviewers: hubert.reinterptrtcast, sfertile, jasonliu, daltenty, Xiangling_L.

Reviewed by: hubert.reinterpretcast, sfertile, daltenty.

Subscribers: DiggerLin, Wuzish, nemanjai, hiraditya, MaskRay, jsji, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67125