[llvm-objcopy] Fix --keep-global-symbol/--globalize-symbol for undefined symbols.

Summary: --keep-global-symbol and --globalize-symbol don't make sense for undefined symbols, so it should be ignored for those symbols. This matches GNU objcopy behavior.

Reviewers: jhenderson, alexshap, jakehehrlich, espindola

Reviewed By: jhenderson, jakehehrlich

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D53733

llvm-svn: 345614