Enable non-fragile ABI for the ObjFW runtime

The following patch enables the non-fragile ABI for the ObjFW runtime.

However, I noticed that it is not possible anymore to disable it with
-fno-objc-nonfragile-abi like it was before. I think this functionality should
be restored, but I guess this is not in scope for 3.4 anymore.

Patch by Jonathan Schleifer!

llvm-svn: 195547