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.
Nice! I've been looking for this option and couldn't find it. Is there an equivalent option for GCC?
Related link: http://www.linuxfromscratch.org/lfs/view/development/chapter05/binutils-pass1.html