[XCOFF][AIX] Emit TOC entries for object file generation

Summary:
Implement emitTCEntry for PPCTargetXCOFFStreamer.
Add TC csects to TOCCsects for object file writing.

Note:

1. I did not include any raw data testing for this object file generation
because TC entries raw data will all be 0 without relocation implemented.
I will add raw data testing as part of relocation testing later.
2. I removed "Symbol->setFragment(F);" for common symbols because we
 don't need it, and if we have it then we would hit assertions below:
Assertion `(SymbolContents == SymContentsUnset ||
            SymbolContents == SymContentsOffset) &&
            "Cannot get offset for a common/variable symbol"' failed.
3.Fixed incorrect TOC-base alignment.

Differential Revision: https://reviews.llvm.org/D70798