cross-stdenv: Only prune most overrides in the final stage

Before all overrides were also pruned in the previous stage, now
only gcc and binutils are, because they alone care about about the
target platform. The rest of the overrides don't, so it's better to
preserve them in order to avoid spurious rebuilds.

pkgs/stdenv/adapters.nix

@@ -58,6 +58,10 @@ rec {
   # builds.
   makeStdenvCross = stdenv: cross: binutilsCross: gccCross: stdenv // {
 
+    # Overrides are surely not valid as packages built with this run on a
+    # different platform.
+    overrides = _: _: {};
+
     mkDerivation =
       { name ? "", buildInputs ? [], nativeBuildInputs ? []
       , propagatedBuildInputs ? [], propagatedNativeBuildInputs ? []