Clear vmthread from thread object at shutdown

[tajila]

Related to: #16659

[tajila]

jenkins test sanity alinux64 jdk21

[tajila]

will investigate the jitserver failures

[gacholio]

This is already done in the java code that runs when a thread ends.

[tajila]

This is already done in the java code that runs when a thread ends.

If there is an exception in that code then the threadref is not guaranteed to be cleared. I thought about updating that code and putting eetop = Thread.NO_REF; in a finally block, however, with Java code its always possible that someone can modify it such that the threadref is not cleared. So I figure the safest thing is to do it natively.

[gacholio]

The java code holds a lock while setting it (not sure if that actually does anything), so the native code is not quite the same.

I would say anyone modifying Thread internals in a way that avoids the finally block gets what they deserve, so I think the java solution would be better (especially considering the native code has introduced failures).

[tajila]

Replaced with ibmruntimes/openj9-openjdk-jdk#679