[llvm-objcopy] Introduce dispatch mechanism based on the input

In this diff we introduce dispatch mechanism based on 
the type of the input (archive, object file, raw binary) 
and the format (coff, elf, macho).
We also move the ELF-specific code into the namespace llvm::objcopy::elf.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D53311

llvm-svn: 345217