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!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195547 91177308-0d34-0410-b5e6-96231b3b80d8