Move Objective-C ports to the v2 GNUstep ABI.

 * Bump the LLVM revision used for GNUstep to 7, the minimum to support
   the new ABI.
 * GNUstep-back does not work with lld, so mark it to use Gold (BFD LD
   doesn't seem able to link Objective-C things).
 * Turn off some annoying debug logs in GNUstep back, which generate
   several messages per second when you move the mouse.  These should
   never have been enabled in a release build anyway.
 * Downgrade Cenon to 4.0.2.  This was the last version to actually work
   with GNUstep (the later ones use XCode >= 5 .xib files, which GNUstep
   can't parse).
 * Update gorm to git head. The current release doesn't work with the
   new Objective-C ABI, but -head has the patches to fix it.
 * Update PikoPixel and add it to the gnustep-app meta-package.
 * Update the three core GNUstep packages to the latest release.
 * Update gnumail and pantomime to the latest release and fix a linking
   error with the new ABI.
 * Update GNUstep FTP to the latest version.

Reviewed by:	bapt (previous version)