24-bit retain count

[Larry Campbell]

Currently gnustep-base raises an exception if the retain count exceeds 24 bits. 
There's a comment there:

  /* I've seen comments saying that some platforms only support up to           
                                                    
   * 24 bits in atomic locking, so raise an exception if we try to              
                                                    
   * go beyond 0xfffffe.                                                        
                                                    
   */

Is this really true? On what platforms? 24 bits is really not a very big 
number. (It is causing me pain right now.)

- lc