[Object] Update ObjectFile::makeTriple for XCOFF

Summary:
When we encounter an XCOFF file, reflect that in the triple information.
In addition to knowing the object file format, we know that the
associated OS is AIX.

This means that we can expect that there is no output difference in the
processing of an XCOFF32 input file between cases where the triple is
left unspecified by the user and cases where the user specifies
`--triple powerpc-ibm-aix` explicitly.

Reviewers: jhenderson, sfertile, jasonliu, daltenty

Reviewed By: jasonliu

Subscribers: wuzish, nemanjai, hiraditya, MaskRay, rupprecht, steven.zhang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77025