Forward -u to the linker on gnutools toolchains

Summary:
The BSDs and Darwin all forward the whole 'u' group, but gcc only
forwards -u so far as I can tell. I only forward -u, since that's a
minimal change, and many people object to magically recognizing and
forwarding linker arguments.

Reviewers: chandlerc, joerg

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4304