[jdk21u] RFR: 8315766: Parallelize gc/stress/TestStressIHOPMultiThread.java test

[Soumadipta Roy]

Backporting the fix for https://bugs.openjdk.org/browse/JDK-8315766 merged as part of https://github.com/openjdk/jdk/pull/15710. https://github.com/openjdk/jdk/commit/edd454b502b9bacde55492820e52655bbac63b89.patch could be cleanly applied.

Below are the test results:
* before_release: **1642.99s user 146.56s system 291% cpu 10:14.77 total**
* after_release: **1613.99s user 125.32s system 1323% cpu 2:11.44 total**
* before_fastdebug: **1214.26s user 54.36s system 200% cpu 10:34.09 total**
* after_fastdebug: **1246.25s user 57.25s system 889% cpu 2:26.55 total**

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Commit messages:
 - Backport edd454b502b9bacde55492820e52655bbac63b89

Changes: https://git.openjdk.org/jdk21u/pull/211/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=211&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315766
  Stats: 22 lines in 1 file changed: 20 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u/pull/211.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/211/head:pull/211

PR: https://git.openjdk.org/jdk21u/pull/211