stdenv: implement crossOverlays

crossOverlays only apply to the packages being built, not the build
packages. It is useful when you don’t care what is used to build your
packages, just what is being built. The idea relies heavily on the
cross compiling infrastructure. Using this implies that we need to
create a cross stdenv.

pkgs/stdenv/native/default.nix

@@ -2,7 +2,7 @@
 , localSystem, crossSystem, config, overlays
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 
 let
   inherit (localSystem) system;