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

This feature implements support for making entries in the exception section
on XCOFF on the direct assembly path using the ".except" pseudo-op. It also
provides functionality to lower entries (comprised of language and reason
codes) into the exception section through the use of annotation metadata
attached to llvm.ppc.trap/trapd/tw/tdw intrinsics. Integrated assembler
support will be provided in another review. https://reviews.llvm.org/D133030
needs to merge first for LIT tests

Reviewed By: shchenz, RKSimon

Differential Revision: https://reviews.llvm.org/D132146