[llvm-objcopy] Refactor ELF-specific config out to ELFCopyConfig. NFC.

Summary:
This patch splits the command-line parsing into two phases:

First, parse cross-platform options and leave ELF-specific options unparsed.

Second, in the ELF implementation, parse ELF-specific options and construct ELFCopyConfig.

Reviewers: espindola, alexshap, rupprecht, jhenderson, jakehehrlich, MaskRay

Reviewed By: alexshap, jhenderson, jakehehrlich, MaskRay

Subscribers: mgorny, emaste, arichardson, jakehehrlich, MaskRay, abrachet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67139

llvm-svn: 372712