RFR (L) 8244997: Convert the JavaThread::_threadObj oop to use OopStorage

[Coleen Phillimore]

Summary: Move the oop and handle releasing it in the service thread.  
Remove Universe::oops_do from callers.

See bug for more details and discussion.  C2 code provided by Erik 
Osterlund and Graal changes provided by Tom Rodriguez.

Tested with tiers 1-8.

bug link https://bugs.openjdk.java.net/browse/JDK-8244997
open webrev at http://cr.openjdk.java.net/~coleenp/2020/8244997.01/webrev

Copyrights fixed, not in this webrev.

Thanks,
Coleen