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

Summary:
Adds support for generating the .data section in assembly files for global variables with a non-zero initialization. The support for writing the .data section in XCOFF object files will be added in a follow-on patch. Any relocations are not included in this patch.

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

Reviewed by: hubert.reinterpretcast

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66154

llvm-svn: 369869