Apply makeOverridable to stdenv

This removes the need for hacks like stdenv.regenerate.  It also
ensures that overrideGCC is now stackable (so ‘stdenv = useGoldLinker
clangStdenv’ works).

pkgs/development/compilers/dev86/default.nix

@@ -2,7 +2,7 @@
 
 stdenv.mkDerivation {
   name = "dev86-0.16.19";
-  
+
   src = fetchurl {
     url = http://www.debath.co.uk/dev86/Dev86src-0.16.19.tar.gz;
     sha256 = "33398b87ca85e2b69e4062cf59f2f7354af46da5edcba036c6f97bae17b8d00e";
@@ -17,7 +17,7 @@ stdenv.mkDerivation {
     ''
       substituteInPlace makefile.in --replace "-O2" "" --replace "-O" ""
     '';
-      
+
   meta = {
     description = "Linux 8086 development environment";
     homepage = http://www.debath.co.uk/;