[yaml2obj][XCOFF] parsing auxiliary symbols.

Summary: The patch adds support for yaml2obj parsing auxiliary
symbols for XCOFF. Since the test cases of this patch are
interdependent with D113825 ([llvm-readobj][XCOFF] dump
auxiliary symbols), test cases of this patch will be committed
after D113825 is committed.

Reviewed By: jhenderson, DiggerLin

Differential Revision: https://reviews.llvm.org/D113552