[JDK21] Add support for JVMTI StopThread

[babsingh]

Previously, StopThread returned JVMTI_ERROR_UNSUPPORTED_OPERATION for
a virtual thread.

In JDK21, StopThread includes support for virtual threads as per the
JVMTI specification:

• Error if a virtual thread is not suspended and not the current
  thread.
• Error if a virtual thread is unomunted since it won't be able to
  throw an asynchronous exception from the current frame.

Related:

• JDK21 serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java#no-vmcontinuations RuntimeException: StopThreadTest failed! #17717
• JDK21 serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java#platform Fatal error: error in JVMTI ResumeThread #17718

Also, StopThread should not clear the
J9_PUBLIC_FLAGS_HALT_THREAD_JAVA_SUSPEND flag. This behaviour is not
documented in the JVMTI specification. It causes incorrect behaviour
for ResumeThread when the following sequence of JVMTI functions are
invoked: SuspendThread -> StopThread -> ResumeThread.

[babsingh]

Also, StopThread should not clear the J9_PUBLIC_FLAGS_HALT_THREAD_JAVA_SUSPEND flag. This behaviour is not
documented in the JVMTI specification. It causes incorrect behaviour for ResumeThread when the following sequence of JVMTI functions are invoked: SuspendThread -> StopThread -> ResumeThread.

Test code associated to the above behaviour:

• StopThreadTest.java#L120-L127
• StopThreadTest.java#L142-L147

Verified locally that StopThreadTest passes with the changes from this PR.

[babsingh]

JDK21 JVMTI Specification for StopThread: https://download.java.net/java/early_access/jdk21/docs/specs/jvmti.html#StopThread

[babsingh]

@gacholio Requesting to review.

[gacholio]

jenkins test sanity zlinux jdk8,jdk21