[llvm-objcopy] Factor out CopyConfig

In this diff we move out CopyConfig from llvm-oobjcopy.cpp into a separate header CopyConfig.h
to enable us (in the future) reuse this class in the other implementations of objcopy (for coff, mach-o).
Additionally this enables us to unload the complexity from llvm-objcopy.cpp a little bit.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D53006

llvm-svn: 344307