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)