top-level: move cross pkgs to overlay

pkgs/top-level/all-packages.nix

@@ -31,11 +31,6 @@ with pkgs;
     then pkgsi686Linux.callPackage
     else throw "callPackage_i686 not supported on system '${stdenv.system}'";
 
-  # Useful attribute of systems to cross compile to.
-  pkgsCross = lib.mapAttrs (n: crossSystem:
-              nixpkgsFun { inherit crossSystem; })
-              lib.systems.examples;
-
   # A stdenv capable of building 32-bit binaries.  On x86_64-linux,
   # it uses GCC compiled with multilib support; on i686-linux, it's
   # just the plain stdenv.

pkgs/top-level/stage.nix

@@ -117,6 +117,12 @@ let
     lib.optionalAttrs allowCustomOverrides
       ((config.packageOverrides or (super: {})) super);
 
+  # Convenient way to reference cross
+  # Used in aliases for now but should not be used in Nixpkgs.
+  cross = self: super: { pkgsCross = lib.mapAttrs (n: crossSystem:
+                                     nixpkgsFun { inherit crossSystem; })
+                                     lib.systems.examples; };
+
   # The complete chain of package set builders, applied from top to bottom.
   # stdenvOverlays must be last as it brings package forward from the
   # previous bootstrapping phases which have already been overlayed.
@@ -127,6 +133,7 @@ let
     trivialBuilders
     splice
     allPackages
+    cross
     aliases
     configOverrides
   ] ++ overlays ++ [