gcc: Do not search /usr/lib for libraries

Add -nostdlib to the link_libgcc spec.
gcc: Reorder the binutils and glibc dependencies
Linuxbrew: Enable --default-names by default

It would be good to add a --without-default-names option.
gcc: Do not use --with-system-zlib for Linuxbrew
gcc: Add gcc, g++, cc, c++ symlinks for Linuxbrew