[llvm-objcopy] Add support for --only-keep/-j and --keep

This change adds support for the --only-keep option and the -j alias as well.
A common use case for these being used together is to dump a specific section's
data. Additionally the --keep option is added (GNU objcopy doesn't have this)
to avoid removing a bunch of things. This allows people to err on the side of
stripping aggressively and then to keep the specific bits that they need for
their application.

Differential Revision: https://reviews.llvm.org/D39021

llvm-svn: 319467