[llvm-objcopy][COFF] Add support for extended relocation tables

Summary: This patch adds support for COFF objects with extended relocation tables to the llvm-objcopy tool.

Reviewers: jhenderson, MaskRay, mstorsjo, alexshap, rupprecht

Reviewed By: mstorsjo

Subscribers: jakehehrlich, abrachet, seiya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70205