binutils on darwin: Clean up the rats nest

 - No more *Cross duplication for binutils on darwin either.
   `cctools_cross` is merged into plain `cctools`, so `buildPackages`
   chains alone are used to disambiguate.

 - Always use a mashup of cctools and actual GNU Binutils as `binutils`.
   Previously, this was only done in the native case as nobody had
   bothered to implement the masher in the cross case. Implemented it
   basically consisted of extending the wrapper to deal with prefixed
   binaries.