[llvm-objcopy][NFC] Refactor output target parsing

Summary:
Use an enum instead of string to hold the output file format in Config.InputFormat and Config.OutputFormat. It's essential to support other output file formats other than ELF.

Reviewers: espindola, alexshap, rupprecht, jhenderson

Reviewed By: rupprecht, jhenderson

Subscribers: jyknight, compnerd, emaste, arichardson, fedor.sergeev, jakehehrlich, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63239

llvm-svn: 364254