RFR: 8350765: Need to pin when accessing thread container from virtual thread

[Alan Bateman]

Data structures that are accessed by both virtual threads and their carriers require the virtual thread to pin the continuation to avoid potential deadlock. A deadlock can arise when a virtual thread is preempted, is selected and scheduled to be the next owner of the lock/resource, but can't execute because all carriers are blocking on the same lock/resource. There are a small number of places that need to pin. One that was missed is the the notification to the thread container when threads are started or terminate. This is not currently an issue at this time but it is a potential hazard for ongoing and future work that will add further scheduling points to the code.  Continuation.pin/unpin have intrinsics since JDK-8338745, and Continuation is initialized early in startup. Finally, the changes have been in the loom repo for several months with no issues.

Testing tier1-5, quick statup/footprint, noreg-hard

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Commit messages:
 - No need to used qualified name
 - Merge branch 'master' into JDK-8350765
 - Update
 - Merge branch 'master' into JDK-8350765
 - Add method to ContinuationSupport
 - Missing /
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/23810/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23810&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350765
  Stats: 92 lines in 6 files changed: 48 ins; 25 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/23810.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23810/head:pull/23810

PR: https://git.openjdk.org/jdk/pull/23810