RFR: 8200697: Add utility for spin wait with fallback to yield/sleep

[Kim Barrett]

> On Apr 4, 2018, at 7:42 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> Hi Kim,
> 
> If this works you for then by all means use it.

Thanks.

> Whether it is useful as a general utility is something I am doubtful of.  Degrading spin-loops are put in for performance reasons and generally have to interact with surrounding code that captures exactly what is being waited upon. There is also the issue of thread-state transitions that need to be carefully managed.

Agreed that this is of limited use.  But since Robbin and I came up
with similar approaches, and both were based on examples already in
HotSpot, we decided to package it up.