Add support for objc_msg_lookup_stret.

The assembly implementations of the lookup and forwarding still need to
be adjusted. Those just alias to the non-stret version for now so that
linking works, meaning the behaviour is still the same as without
support for objc_msg_lookup_stret for now.
Add objc_msg_lookup_stret for AMD64/ELF.
Add of_forward_stret for AMD64/ELF.
Add objc_msg_lookup_stret for AMD64/Mach-O.