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.
No, GCC does not take `--with-lib-path`. It does take `--with-local-prefix`, but that only affects searching `/usr/local/include` for headers. It does not affect the library search path.