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/os-specific/linux/kernel-headers/3.18.nix

@@ -35,7 +35,7 @@ stdenv.mkDerivation {
   buildInputs = [perl];
 
   # FIXME needs gcc 4.9 in bootstrap tools
-  hardening_stackprotector = false;
+  hardeningDisable = [ "stackprotector" ];
 
   extraIncludeDirs =
     if cross != null then