stdenv cross adaptor: Add optional overrides parameter

By default, all previous overrides are discarded as before, as they
would only apply to the old host platform. But sometimes it is useful to
add some new ones, and this optional parameter allows that.

pkgs/stdenv/adapters.nix

@@ -59,17 +59,17 @@ rec {
   makeStdenvCross = { stdenv
                     , cc
                     , buildPlatform, hostPlatform, targetPlatform
+                    , # Prior overrides are surely not valid as packages built
+                      # with this run on a different platform, so disable by
+                      # default.
+                      overrides ? _: _: {}
                     } @ overrideArgs: let
     stdenv = overrideArgs.stdenv.override {
       inherit
         buildPlatform hostPlatform targetPlatform
-        cc;
+        cc overrides;
 
       allowedRequisites = null;
-
-      # Overrides are surely not valid as packages built with this run on a
-      # different platform.
-      overrides = _: _: {};
     };
   in stdenv // {
     mkDerivation =