[llvm-readobj][xcoff] implement parsing overflow section header.

SUMMARY:
in the xcoff, if the number of relocation entries or line number entries is
overflow(large than or equal 65535) , there will be overflow section for it.
The interpret of overflow section is different with generic section header,
the patch implement parsing the overflow section.

Reviewers: hubert.reinterpretcast,sfertile,jasonliu
Subscribers: rupprecht, seiya

Differential Revision: https://reviews.llvm.org/D68575

llvm-svn: 374941