binutils: Fix cross build

Broken after commit 17a344a ("binutils: Add lib output").

pkgs/development/tools/misc/binutils/default.nix

···
   ];
 
   outputs = [ "out" ]
-    ++ optional (!stdenv.isDarwin) "lib" # problems in Darwin stdenv
+    ++ optional (cross == null && !stdenv.isDarwin) "lib" # problems in Darwin stdenv
     ++ [ "info" ]
     ++ optional (cross == null) "dev";