[jdk21u-dev] RFR: 8327501: Common ForkJoinPool prevents class unloading in some cases

[Aleksey Shipilev]

The change is stabilizing in mainline, but it looks simple, so we start testing it for JDK 21 pickup here.

Additional testing:
 - [ ] Linux x86_64 server fastdebug, `all`

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Commit messages:
 - Backport 53c4714aab2e072ba18631875dcaa3b2d5d22243

Changes: https://git.openjdk.org/jdk21u-dev/pull/338/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=338&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327501
  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/338.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/338/head:pull/338

PR: https://git.openjdk.org/jdk21u-dev/pull/338