aliases: add forceSystem

Some uses of this still exist outside of Nixpkgs

pkgs/top-level/aliases.nix

@@ -319,6 +319,11 @@ mapAliases ({
   libva-full = libva;
   libva1-full = libva1;
 
+  # forceSystem should not be used directly in Nixpkgs.
+  # added 2018-07-16
+  forceSystem = system: _:
+    (import self.path { localSystem = { inherit system; }; });
+
   inherit (ocaml-ng) # added 2016-09-14
     ocamlPackages_3_10_0 ocamlPackages_3_11_2 ocamlPackages_3_12_1
     ocamlPackages_4_00_1 ocamlPackages_4_01_0 ocamlPackages_4_02