RFR: 8218727: vmTestbase/nsk/jvmti/scenarios/events/EM04/em04t001/TestDescription.java crash in native library

[Daniil Titov]

Please review the change that fixes the test when running with Graal on.

The problem here is the lack of synchronization when accessing the internal data structure (the list of events) in the agent procedure and in the callbacks the test enables. That results in the agent proc starts removing elements from the list of the events while the callback handlers are still enabled, assuming that no new events could be generated,  but in case of Graal it is not true. This causes to the crash when the callback iterates over the list of events and tries to access the element that was just removed by the agent procedure. 

Webrev: http://cr.openjdk.java.net/~dtitov/8218727/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8218727

Thanks!

--Daniil