top-level: turn the screw

- Non-cross stdenvs are honest and assert that `crossSystem` is null

 - `crossSystem` is a mandatory argument to top-level/stage.nix, just like
   `system` and `platform`

 - Broken default arguments on stdenvs for testing are gone.

 - All stdenvs (but little-used stdenvNix) take the same arguments for easy
   testing.

pkgs/stdenv/native/default.nix

@@ -1,4 +1,8 @@
-{ system, allPackages ? import ../../.., config }:
+{ lib, allPackages
+, system, platform, crossSystem, config
+}:
+
+assert crossSystem == null;
 
 rec {
 
@@ -126,7 +130,7 @@ rec {
   } // {inherit fetchurl;};
 
   stdenvBoot1Pkgs = allPackages {
-    inherit system platform config;
+    inherit system platform crossSystem config;
     allowCustomOverrides = false;
     stdenv = stdenvBoot1;
   };