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/linux/default.nix

@@ -3,11 +3,9 @@
 # external (non-Nix) tools, such as /usr/bin/gcc, and it contains a C
 # compiler and linker that do not search in default locations,
 # ensuring purity of components produced by it.
+{ lib, allPackages
+, system, platform, crossSystem, config
 
-# The function defaults are for easy testing.
-{ system ? builtins.currentSystem
-, allPackages ? import ../../..
-, platform ? null, config ? {}, lib ? (import ../../../lib)
 , bootstrapFiles ?
     if system == "i686-linux" then import ./bootstrap/i686.nix
     else if system == "x86_64-linux" then import ./bootstrap/x86_64.nix
@@ -18,6 +16,8 @@
     else abort "unsupported platform for the pure Linux stdenv"
 }:
 
+assert crossSystem == null;
+
 rec {
 
   commonPreHook =
@@ -106,7 +106,7 @@ rec {
       };
 
       thisPkgs = allPackages {
-        inherit system platform config;
+        inherit system platform crossSystem config;
         allowCustomOverrides = false;
         stdenv = thisStdenv;
       };