ParallelGC Full GC should use parallel WeakProcessor

JDK-8210100 only adds parallel weak oops processing to Young GC, but not Full GC.

In `psParallelCompact.cpp`:

maybe the required change is just from

```
WeakProcessor::weak_oops_do(is_alive_closure(), &do_nothing_cl);
```
to
```
WeakProcessor::weak_oops_do(ParallelScavengeHeap::heap()->workers(),
                           is_alive_closure(), &do_nothing_cl, 1);
```

PS: make sure the logging looks OK.