[llvm-objcopy] Move elf-specific code into subfolder

In this diff the elf-specific code is moved into the subfolder ELF 
(and factored out from llvm-objcopy.cpp).

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D53790


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345544 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed