Targeting OS X (was Re: What gcc to use for gdc master?)

[Dan Olson via D.gnu]

Seems to be working good now.  Unittests for druntime all pass so Fiber
switching code is working.  Unittests for phobos fails in biguintcore,
but I don't suppose that is an OS X specific thing.

Iain, I am going to make separate pull requests for each OS X topic
(fibers, symbol mangling, gc data scanning) unless you prefer in one.