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/tools/system/cron/default.nix

@@ -9,7 +9,7 @@ stdenv.mkDerivation {
 
   unpackCmd = "(mkdir cron && cd cron && sh $curSrc)";
 
-  hardening_pie = true;
+  hardeningEnable = [ "pie" ];
 
   preBuild = ''
     substituteInPlace Makefile --replace ' -o root' ' ' --replace 111 755