RFR: 8350765: Need to pin when accessing thread container from virtual thread [v2]

[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

Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - Fix/improve comment
 - Merge branch 'master' into JDK-8350765
 - Merge branch 'master' into JDK-8350765
 - 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

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23810/files
  - new: https://git.openjdk.org/jdk/pull/23810/files/2959e070..5ff93aea

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23810&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23810&range=00-01

  Stats: 2069 lines in 129 files changed: 1811 ins; 61 del; 197 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