[8u] RFR (S) 8227018: CompletableFuture should not call Runtime.availableProcessors on fast path

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8227018

8u webrev:
  http://cr.openjdk.java.net/~shade/8227018/webrev.01/

In 8u, CompletableFuture calls Runtime.availableProcessors during spin in waitingGet. Unfortunately,
that falls victim to JDK-8227006, which makes it too costly. This is fixed in 9 with JDK-8134851,
but that patch is too large and intrusive to backport to 8u. Therefore, we should consider
backporting the small part of that patch that deals with availableProcessors handling.

Testing: "tier1"-like testing

-- 
Thanks,
-Aleksey