[jdk26] RFR: 8375130: [BACKOUT] Scalability issue when submitting virtual threads with almost empty tasks

[Alan Bateman]

This PR is for the jdk26 branch, not main line.

The changes in [JDK-8360046](https://bugs.openjdk.org/browse/JDK-8360046) improve the scalability of virtual threads but introduce a subtle issue with signal filtering and a regression in some SPECjbb2015 runs. [pull/28797](https://git.openjdk.org/jdk/pull/28797) is in progress for main line to address these issues.

For the jdk26 branch, the proposal is to backout the changes to ForkJoinPool from JDK-8360046 but leave the small change to VirtualThread to submit(ForkJoinTask) consistently.  As part of the change, the acquire fence change to address [JDK-8372835](https://bugs.openjdk.org/browse/JDK-8372835) are proposed to be included, to avoid bringing back an issue that was fixed by JDK-8360046. So a somewhat complicated story due to jdk26 being in RDP1.


Testing: test1-5, repeat testing of java/util/concurrent and java/lang/Thread/virtual tests.

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Commit messages:
 - Backout JDK-8360046, incorporate JDK-8372835

Changes: https://git.openjdk.org/jdk/pull/29187/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29187&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375130
  Stats: 470 lines in 2 files changed: 143 ins; 143 del; 184 mod
  Patch: https://git.openjdk.org/jdk/pull/29187.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29187/head:pull/29187

PR: https://git.openjdk.org/jdk/pull/29187