[llvm-readobj][XCOFF] Add support for `--needed-libs` option.

Summary: This patch is trying to add support for llvm-readobj
--needed-libs option under XCOFF.
For XCOFF, the needed libraries can be found from the Import
File ID Name Table of the Loader Section.
Currently, I am using binary inputs in the test since yaml2obj
does not yet support for writing the Loader Section and the
import file table.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D106643