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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319467 91177308-0d34-0410-b5e6-96231b3b80d8
