RFR: Add support for CollectedHeap::get_safepoint_workers()

[Per Liden]

Added support for CollectedHeap::get_safepoint_workers(), to reduce 
safepoint cleanup times.

http://cr.openjdk.java.net/~pliden/zgc/get_safepoint_workers/webrev.0

Here are some numbers on from SPECjbb2015, which look encouraging.

Before:

Safepoint cleanup tasks (secs):
                 avg: 0.00286622 (+/-0.00189263)
     95th percentile: 0.0059704
     99th percentile: 0.006819
   99.9th percentile: 0.0089186
  99.99th percentile: 0.0117573
                 max: 0.0117573

After:

Safepoint cleanup tasks (secs):
                 avg: 0.000676487 (+/-0.000212009)
     95th percentile: 0.0010262
     99th percentile: 0.0011429
   99.9th percentile: 0.0012933
  99.99th percentile: 0.0013363
                 max: 0.0013363

/Per