[llvm-objcopy] - Refactor the code. NFC.

The idea of the patch is about to move out the code to a new
helper static functions (to reduce the size of 'handleArgs' and to
isolate the parts of it's logic).

Differential revision: https://reviews.llvm.org/D59762

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356889 91177308-0d34-0410-b5e6-96231b3b80d8