GNUstep with the new objc runtime and clang. For now, gnustep uses the old gnu objc runtime and gcc on ppc 32 and 64. But this runtime doesn't offer the "modern" features of the language (automatic reference counting, blocks...) known as Objc2. GNUstep offers a new objc runtime for years now, which brings this features to linux objc. But this runtime has to be build with clang.
The last time I tried on Debian Jessie, it failed. Tweaks are needed and also a small amount of lines of ppc assembly to optimize some function of the runtime.
I'd be very happy to be helped on this, and the gnustep project also.