gcc: patch

TL;DR - Patches gcc so it compiles on Yosemite.

Long version - I've spent way too many hours testing this to hell and back, and I'm pretty content with the proposed patch at this point. Brew audit turns up no errors or complications, and I've successfully compiled, tested & used this patched gcc inside Homebrew on OS X 10.10 three times in a row.

This could be controversial though, because to do so I've removed the current ``` --HEAD ``` branch. Part of my justification for this is that what I've changed it to is the actual ``` trunk ``` on the gcc git, which is where the Homebrew Formula cookbook says ``` --HEAD ``` should be pointed.

I believe that I have scoped the patch to the trunk for only Yosemite, which will ensure the ``` --HEAD ``` option doesn't break for other OS X versions.

Completely aware this patch is likely to be controversial though, and may well end up being rejected, both of which I will respect if happen. I thought I'd throw this in as a proposed patch nonetheless, and allow others to discuss the merits of it.