[Nix-dev] gcc for MacOS X

[Ludovic Courtès]

Hello!

Here we are!  ‘stdenvNative’ on Darwin now uses GCC 4.6, like on all the
other platforms.  The Darwin-specific tools (‘arch’, ‘sw_vers’, etc.)
are now part of ‘stdenv’ as well, and ‘darwinArchUtility’ & co. are gone.

Let me know if there’s anything wrong!

Ludo’.