Stop binutils from trying to build on Darwin

Sometimes, for no reason whatsoever, my machine tries to build binutils. This happens whenever a package overrides `stdenv` to use `gccApple` (examples: redis, phantomjs). I've spoken to some other Darwin users in #nixos and some have the same problem, but some (@jwiegley as far as I know) don't.

Until someone, possibly me, can figure out the problem, this is a patch that'll unconditionally stop binutils from building on Darwin and also hopefully restricts any references to binutils to use native-darwin-cctools-wrapper.