gcc, binutils: link to Cellar instead of system libs

Symlinks `linuxbrew/lib` to Cellar for `gcc`, where `gcc` will automatically look because that is the prefix. Should fix the issue discussed recently in #107.
It does seem a little strange, but what I figured out was that the libs need to be passed to `xgcc` in `-B` options, and this is one of the default locations. I couldn't figure out a way to control what was passed to `xgcc`.