kernel - Improve exec performance

kernel - Improve exec performance

* Improves non-shared 32-way-concurrent exec performance for a small
  static binary on the xeon from 92KE/s (92000 execs/sec) to 136KE/s.

* Improves single-threaded test performance from ~4.5KE/s to ~6.5KE/s.
  And for reasons I don't entirely understand, sometimes up to ~8KE/s.

* Several changes here, but the only one that matters for the test is
  that the pv_placemarker_wakeup() code removes a spin_lock/spin_unlock
  pair on the pmap.  I adjusted the code so the pmap spinlock is not
  required for placemarker wakeup operations.

  What I think might have happened here is that this removal also got
  rid of a spin-lock shared/exclusive ping-pong.  Still, the huge
  improvement in performance was not expected.  Even with the removal
  there is still an atomic_swap_long() in the code path.

  My guess is that multiple atomic ops degrade the instruction pipeline
  more than one would otherwise expect due to the multiple memory
  fences.