kernel - Fix very annoying lockup (SMP)

kernel - Fix very annoying lockup (SMP)

* Fix an extremely annoying lockup that took a week+ to find.  The cpusync
  code is rather fragile and any for (;;) or while (1) style loops in the
  kernel can trip it up and cause a deadlock.  These loops are careful to
  call lwkt_process_ipiq() to ensure that cpusync processing occurs.

  However, there is a race in the LWKT thread migration code where a thread
  deschedules itself on one cpu and schedules itself on another via a remote
  ipi.  The target cpu expects the thread's TDF_RUNNING state to clear and
  will loop until that happens.

  An IPI could sneak itself into the deschedule/lwkt_switch() path and
  deadlock against a cpusync, preventing the thread from leaving the
  TDF_RUNNING state.

  The solution is to ensure that lwkt_process_ipiq() is *NOT* run in
  the lwkt_switch() path if the calling thread has descheduled itself.

* The original bug could be reproduced by running blogbench in one window
  and a while (1) ps axl shell script in another.

* Add DEBUG_PUSH_INFO(msg)/DEBUG_POP_INFO() macros which record (msg)
  in the globaldata structure as a debugging aid.

* Remove unused platform/pc64/x86_64/systimer.c file.  The entire contents
  of this file was #ifdef'd out and its functionality is handled elsewhere
  by the lapic timer code.

* #if 0 out numerous debugging bits but don't remove the code because it
  is extremely useful for finding lockup conditions.