[llvm-objcopy] Factor out Buffer

In this diff we move out the hierarchy of buffers from Object.h/Object.cpp 
into separate files since it is not ELF-specific and will be reused later. 
After this change Object.h/Object.cpp are almost exclusively ELF-specific.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D53298

llvm-svn: 344585