These tools are based on the recommended setups in osdev.org
To use:
nix-build -f cross-i686.nix -A { i686-gcc | i686-binutils }
nix-env -f cross-i686.nix -iA { i686-gcc | i686-binutils }
You don't need to build the binutils explicitly if you build gcc, but binutils will be needed later.