[AIX] llvm-readobj support a new option --exception-section for xcoff object file.

Summary:

llvm-readobj support a new option --exception-section for xcoff object file.

https://www.ibm.com/docs/en/aix/7.2?topic=formats-xcoff-object-file-format#XCOFF__iua3i23ajbau

Reviewers:  James Henderson,Paul Scoropan

Differential Revision: https://reviews.llvm.org/D133030