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/freebsd/default.nix

@@ -1,9 +1,9 @@
-{ system      ? builtins.currentSystem
-, allPackages ? import ../../..
-, platform    ? null
-, config      ? {}
+{ lib, allPackages
+, system, platform, crossSystem, config
 }:
 
+assert crossSystem == null;
+
 rec {
   inherit allPackages;