kernel - Reoptimize sys_pipe

kernel - Reoptimize sys_pipe

* Use atomic ops for state updates, allowing us to avoid acquiring
  the other side's token.  This removes all remaining contention.

* Performance boosted by around 35%.  On the ryzen, bulk buffer
  write->read tests between localized cpu cores went from 9.2 GB/sec
  to around 13 GBytes/sec.  Cross-die performance increased from
  2.5 GB/sec to around 4.5 GB/sec (gigabytes/sec).

  1-byte ping-ponging (write-1/read-1/turn-around/write-back-1/
  read-back1) fell from 1.0-2.0uS to 0.7uS to 1.7uS.

* Add kern.pipe.size, allowing the kernel pipe buffer size to be
  changed (effects new pipes only).  The default buffer size has
  been increased to 32KB (it was 16KB).

* Refactor pipelining optimizations, further reducing unnecessary
  tsleep/wakeup IPIs.

* Improve kern.pipe.delay operation (an IPI avoidance mechanism),
  and reduce from 5uS to 4uS.

  Also add cpu_pause() in the TSC loop (suggested-by mjg_).