llvm(Packages) on darwin: attempts to fix

- mark llvm34 as broken on darwin (so it doesn't install by default with nix-env)
- don't use our gcc for llvm_34 (might fix the build)
- switch also clang default to 3.3 on darwin (llvm was before)

pkgs/development/compilers/llvm/3.4/llvm.nix

@@ -54,5 +54,6 @@ in stdenv.mkDerivation rec {
     license     = stdenv.lib.licenses.bsd3;
     maintainers = with stdenv.lib.maintainers; [ shlevy lovek323 raskin viric ];
     platforms   = stdenv.lib.platforms.all;
+    broken      = stdenv.isDarwin;
   };
 }