Provide easier access to configuring the linker

I've got a fairly big project that I work on, and I've been investigating how to speed up builds. One thing that used to be recommended was using the gold linker on supported platforms rather than the traditional GNU linker.

It's not clear to me what linker GHC chooses to use on modern Linux releases at the moment (Ubuntu, 16.10), so please disregard if this GHC already favors the gold linker out of the box. But if not.. it would be great to have have a stanza in the yaml file to allow specifying the preferred linker and linker options.