[llvm-objcopy] Add -F|--target compatibility

Summary:
This change adds support for the GNU --target flag, which sets both --input-target and --output-target.

GNU objcopy doesn't do any checking for whether both --target and --{input,output}-target are used, and so it allows both, e.g. "--target A --output-target B" is equivalent to "--input-target A --output-target B" since the later command line flag would override earlier ones. This may be error prone, so I chose to implement it as an error if both are used. I'm not sure if anyone is actually using both.

Reviewers: jakehehrlich, jhenderson, alexshap

Reviewed By: jakehehrlich, alexshap

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53029

llvm-svn: 344321