RFR: 8297286: runtime/vthread tests crashing after JDK-8296324

[Serguei Spitsyn]

This problem has two sides.
One is that the `VirtualThread::run() `cashes the field `notifyJvmtiEvents` value.
It caused the native method `notifyJvmtiUnmountBegin()` not called after the field `notifyJvmtiEvents`
value has been set to `true` when an agent library is loaded into running VM.
The fix is to get rid of this cashing.
Another is that enabling `notifyJvmtiEvents` notifications needs a synchronization.
Otherwise, a VTMS transition start can be missed which will cause some asserts to fire.
The fix is to use a JvmtiVTMSTransitionDisabler helper for sync.

Testing:
The originally failed tests are passed now:

runtime/vthread/RedefineClass.java
runtime/vthread/TestObjectAllocationSampleEvent.java 

In progress:
Run the tiers 1-6 to make sure there are no regression.

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Commit messages:
 - 8297286: runtime/vthread tests crashing after JDK-8296324

Changes: https://git.openjdk.org/jdk/pull/11304/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11304&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297286
  Stats: 6 lines in 3 files changed: 1 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11304.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11304/head:pull/11304

PR: https://git.openjdk.org/jdk/pull/11304