Named Thread: introduce print() and print_on(outputStream* st) methods

The NamedThread class extends the Thread class with name field. However, it doesn't introduce print() / print_on(outputStream* str) methods to be used when doing a thread dump. As a result, we replicate code like this:

void ConcurrentMarkSweepThread::print_on(outputStream* st) const {
  st->print("\"%s\" ", name());
  Thread::print_on(st);
  st->cr();
}

It'd be nice to avoid this replication by moving this functionality to the NamedThread class.