[llvm-objcopy] Support the rest of the ELF formats

We haven't been supporting anything but ELF64LE since the start. Luckily
this was always accounted for and the change is pretty trivial. B35281
requests this change for ELF32LE. This change adds support for ELF32LE,
ELF64BE, and ELF32BE with all supported features that already existed
for ELF64LE.

Differential Revision: https://reviews.llvm.org/D39977

llvm-svn: 318166