RFR: Parallelize safepoint cleanup

[Kim Barrett]

> On May 29, 2017, at 4:23 AM, Roman Kennke <rkennke at redhat.com> wrote:
> I implemented this using the following:
> 
>  if (ParallelSafepointCleanup &&
> FLAG_IS_DEFAULT(ParallelSafepointCleanupThreads)) {
>    // This will pick up ParallelGCThreads if set...
>    FLAG_SET_DEFAULT(ParallelSafepointCleanupThreads,
> Abstract_VM_Version::parallel_worker_threads());
>  }

Just following along, and not presently planning to do a real review, but the
above caught my eye.  Clients of the VM_Version infrastructure should never
directly reference Abstract_VM_Version::<anything>, as doing so may bypass
platform-specific specializations.

I thought I’d filed a bug about that sort of thing, but apparently that got pushed
deep onto my todo list.  I’ll go take care of that now.