[llvm-objcopy] - Do not drop the OS/ABI and ABIVersion fields of ELF header

This is https://bugs.llvm.org/show_bug.cgi?id=40005,

Patch teaches llvm-objcopy to preserve OS/ABI and ABIVersion fields of ELF header.
(Currently, it drops them to zero).

Differential revision: https://reviews.llvm.org/D55886

llvm-svn: 349738