RFR: 8304448: Kitchensink failed: assert(!thread->is_in_any_VTMS_transition()) failed: class prepare events are not allowed in any VTMS transition

[Serguei Spitsyn]

The fix was suggested/provided by @AlanBateman.
Continuation should use Unsafe.compareAndSetReference rather than a VH here.
We had to change CHM and VT back to Unsafe for similar reasons.

Testing: Mach5 runs of tiers 1-6 are in progress

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Commit messages:
 - 8304448: Kitchensink failed: assert(!thread->is_in_any_VTMS_transition()) failed: class prepare events are not allowed in any VTMS transition

Changes: https://git.openjdk.org/jdk/pull/13154/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304448
  Stats: 14 lines in 1 file changed: 1 ins; 10 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13154.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13154/head:pull/13154

PR: https://git.openjdk.org/jdk/pull/13154