Add test for interaction of --gc-sections and undefined references

It is possible for a link to fail with an undefined reference, unless
--gc-sections is specified, removing the reference in the process. This
doesn't look to be tested anywhere explicitly, so I thought it useful
to add a test for it to ensure the behaviour is maintained.

Reviewers: ruiu

Differential Revision: https://reviews.llvm.org/D42299

llvm-svn: 323099