RFR (S) 8240440: Implement get_safepoint_workers() for parallel GC

[Schmelter, Ralf]

Hi,

could you review the small change. It implements get_safepoint_workers() for the ParallelScavengeHeap, so that the worker threads could be used for other tasks. This is already implemented for G1, Z and Shenandoah. Since the parallel GC does used the worker threads only in the collection VM operation it can safely share them.

bugreport: https://bugs.openjdk.java.net/browse/JDK-8240440
webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8240440/webrev.0/

Best regards,
Ralf