Add a define for the ObjFW runtime ABI version.

This removes __has_feature(objc_msg_lookup_stret), as it is not required
anymore after this patch.

Patch by Jonathan Schleifer!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190791 91177308-0d34-0410-b5e6-96231b3b80d8