Enable assembly output of local commons for AIX

Summary:
This patch enable assembly output of local commons for AIX using .lcomm
directives. Adds a EmitXCOFFLocalCommonSymbol to MCStreamer so we can emit the
AIX version of .lcomm assembly directives which include a csect name. Handle the
case of BSS locals in PPCAIXAsmPrinter by using EmitXCOFFLocalCommonSymbol. Adds
a test for generating .lcomm on AIX Targets.

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

Reviewed By: sfertile

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64825

llvm-svn: 368306