RFR: 8215487: ZGC: ZRuntimeWorkers incorrectly identify themselves as ZWorkers

[Per Liden]

ZThread::is_worker() incorrectly returns true when executed from a 
ZRuntimeWorkers context. This function should only return true when 
executed from a ZWorker context. This issue can lead to data corruption 
as a ZRuntimeWorker might read and write to data structures that are 
thread local to a ZWorker.

With this patch, the ZRuntimeWorkers will execute an initialize task 
(similar to what ZWorkers already do) to initialize the "worker type" data.

Bug: https://bugs.openjdk.java.net/browse/JDK-8215487
Webrev: http://cr.openjdk.java.net/~pliden/8215487/webrev.0

/Per