Implement OnSpinWait based on WFET - wait for event with timeout:

 - introduce OnSpinWaitDelay - the OnSpinWait time in nanoseconds;
 - the OnSpinWaitInstCount is expected to be 1 when WFET is used;
 - the waiting loop is followed by SB - to ensure following instructions
   aren't speculated until wait is finished;
 - the timer register is read via the self-synchronized view
   CNTVCTSS_EL0 to prevent the read being hoisted out of the loop.