RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v2]

[Patricio Chilano Mateo]

On Wed, 6 Nov 2024 15:57:55 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> The two extension events were designed to be posted when the current thread identity is virtual, so this behavior > needs to be considered as a bug. My understanding is that it is not easy to fix.
>
If we want to post the mount/unmount events here is actually simple if we also use `JvmtiVTMSTransitionDisabler::VTMS_vthread_mount/unmount`. I included it in the last commit.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1831899882