Use general hardening flag toggle lists

The following parameters are now available:

  * hardeningDisable
    To disable specific hardening flags
  * hardeningEnable
    To enable specific hardening flags

Only the cc-wrapper supports this right now, but these may be reused by
other wrappers, builders or setup hooks.

cc-wrapper supports the following flags:

  * fortify
  * stackprotector
  * pie (disabled by default)
  * pic
  * strictoverflow
  * format
  * relro
  * bindnow

pkgs/shells/dash/default.nix

@@ -8,7 +8,7 @@ stdenv.mkDerivation rec {
     sha256 = "03y6z8akj72swa6f42h2dhq3p09xasbi6xia70h2vc27fwikmny6";
   };
 
-  hardening_format = false;
+  hardeningDisable = [ "format" ];
 
   meta = {
     homepage = http://gondor.apana.org.au/~herbert/dash/;