[PowerPC] XCOFF exception section support on the integrated assembler path

[PowerPC] XCOFF exception section support on the integrated assembler path

Continuation of https://reviews.llvm.org/D132146 (direct assembly path support, needs to merge first). Adds support to the integrated assembler path for emitting XCOFF exception sections. Both features need https://reviews.llvm.org/D133030 to merge first

Reviewed By: shchenz, DiggerLin

Differential Revision: https://reviews.llvm.org/D134195