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's also relate to the `GCC_EXEC_PREFIX` environment variable: https://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html#Environment-Variables