This fixes the out of memory errors we were getting on sparc
   after the following patch:

     Fri Mar 13 03:45:16 PDT 2009  Simon Marlow <marlowsd@gmail.com>
     * Instead of a separate context-switch flag, set HpLim to zero
     Ignore-this: 6c5bbe1ce2c5ef551efe98f288483b0
     This reduces the latency between a context-switch being triggered and
     the thread returning to the scheduler, which in turn should reduce the
     cost of the GC barrier when there are many cores.