[objc-gnustep] Don't emit .guess ivar offset vars.

These were intended to allow non-fragile and fragile ABI code to be
mixed, as long as the fragile classes were higher up the hierarchy than
the non-fragile ones.  Unfortunately:

 - No one actually wants to do this.
 - Recent versions of Linux's run-time linker break it.

llvm-svn: 339128