RFR: 8296896: Change virtual Thread.yield to use external submit

[Alan Bateman]

The implementation of Thread.yield for virtual threads is currently a "lazy submit". This means the task for the virtual thread is queued to the carrier/worker local queue without signalling other threads. This behavior can be surprising/unfair when there are tasks in the submission queues, say when a platform thread has started or unparked a virtual thread.

Ron, Doug Lea, Viktor Klang and I have discussed this topic and propose to change Thread.yield to  use "external submit" when the local task queue is empty, and to push to the local queue when not empty. The change improves the fairness but will of course increase the chances that repeated Thread.yield will bounce between carriers.

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Commit messages:
 - Reduce instanceof checks
 - Update
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/11533/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11533&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296896
  Stats: 188 lines in 3 files changed: 161 ins; 12 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/11533.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11533/head:pull/11533

PR: https://git.openjdk.org/jdk/pull/11533