kernel - Add atomic ops features

kernel - Add atomic ops features

* Add <atomicop>_xacquire() and <atomicop>_xrelease() to a bunch of
  atomic ops.  Using this API will elide the operation using Intel's
  TSX extensions (the elides are backwards compatible to cpus that do
  not support TSX).

* Note however that preliminary testing using this API in the spin_lock*()
  and pv_*() interfaces has only led to a reduction of performance and not
  to any improvement, so at the moment we do not intend to use it.  But I
  did the work to add the API calls and I don't want to lose it.