Summary: Start ServiceThread before compiler threads, and run nmethod
barriers for zgc before adding to the service thread queue, or posting
the events on the java thread queue.
See bug for description of the problems found with the new Zombie.java test.
open webrev at http://cr.openjdk.java.net/~coleenp/2019/8235829.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8235829
Ran tier1 all platforms, and tier2-8 testing, as well as rerunning
original test failure from bug
https://bugs.openjdk.java.net/browse/JDK-8173361.
Thanks,
Coleen
