[llvm-objcopy] Strip debug sections when running with --strip-unneeded.

Summary:
GNU --strip-unneeded strips debugging sections as well. Do that for llvm-objcopy as well.

Additionally, add a test that verifies we keep the .gnu_debuglink section. This apparently was not always the case, and I'm not sure which commit fixed it, but there doesn't appear to be any test coverage to make sure we continue to do so.

This fixes PR41043.

Reviewers: jhenderson, jakehehrlich, espindola, alexshap

Subscribers: emaste, arichardson, MaskRay, abrachet, seiya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66623

llvm-svn: 369761