missing CAS-locks and barriers

missing CAS-locks and barriers

 * %WAITQUEUE-ENQUEUE was missing the CAS-lock, as was checking for
   the wakeup.

 * Put back the spin-before-yielding loop into WITH-CAS-LOCK, which
   I'd removed for some reason.

 * PPC has threads, and really needs barriers. x86oids have made me
   lazy, and now we pay the price. In particular:

   ** THREAD-WAITING-FOR: a read barrier in the non-futex
      CONDITION-WAIT -- the corresponding writes are protected
      by CAS and hence provide a write barrier already.

      ...and just for symmetry and because this makes my poor
      head hurt add write and read barriers to other places where
      it is read from / written to.

   ** WITH-CAS-LOCK: a read barrier for the READ-FORM. Not strictly
      necessary perhaps, as THREAD-YIELD most probably provides a
      barrier, but this is easier to read. Since the corresponding
      writes should use CAS, we're OK.