RFR: 8152196: SuspendibleThreadSet::yield scales poorly

[Kim Barrett]

Please review this fix for a performance scaling problem in
SuspendibleThreadSet, leading to unnecessary additional safepoint
latency as the number of suspendible threads increases. See the CR for
details of the problem and some performance data.

CR:
https://bugs.openjdk.java.net/browse/JDK-8152196

Webrev:
http://cr.openjdk.java.net/~kbarrett/8152196/webrev.00/

Testing:
JPRT, RBT GC Nightly, local specjbb2015.