kernel - x86_64 - Add additional checks to lwp_wait() & friends

kernel - x86_64 - Add additional checks to lwp_wait() & friends

* lwp_wait() must defer reaping of a lwp that is still running on another
  cpu (i.e. in the midst of exiting).

* It shouldn't be possible for this to happen but just incase the thread
  gets switched out after TDF_EXITING has been set, also make sure
  the threat is no longer on the LWKT run queue.

* Remove old debugging in the LWKT scheduler path.

* Protect gd_freetd (per-cpu td cache) with a critical section.  Again
  this case should not occur as new threads are not allocated from
  interrupts, but protect it anyway.  Also assert that the cached free
  td is in no way still scheduled.

  Note that this cache is required to ensure that the td does not
  end up in the MP-accessible objcache before it has been fully
  descheduled.