RFR: 8202649: Move the Parallel GC specific task creation functions out of Threads

[Stefan Karlsson]

Hi all,

Please review this patch to move the Parallel GC specific task creation 
functions out of Threads.

http://cr.openjdk.java.net/~stefank/8202649/webrev.01/

There exist (at least) two available functions to iterate over a subset 
of threads:
  static void non_java_threads_do(ThreadClosure* tc);
  static void threads_do(ThreadClosure* tc);

This patch extends this set and adds:
  static void java_threads_do(ThreadClosure* tc);
  static void java_threads_and_vm_thread_do(ThreadClosure* tc);

The latter is used to implement the create_thread_roots_tasks and 
create_thread_roots_marking_tasks inside the Parallel GC code.

Thanks,
StefanK