[NFC] [XCOFF] Update PowerPC readobj test case with expression

This patch is to replace the fixed value with expression.
Keep .file section as fixed values as it might be changed. The
remaining sections will hardly be modified. So the Index values
are sequential. By using expression, we can avoid the fixed value
changes in coming patches.

This is a follow-up of patch D97117.

Reviewed By: hubert.reinterpretcast, shchenz

Differential Revision: https://reviews.llvm.org/D98620