[jdk11u-dev] 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: **1535.51s user 72.40s system 257% cpu 10:24.79 total**
* after_release: **1655.73s user 74.16s system 1195% cpu 2:24.68 total**
* before_fastdebug: **1064.03s user 57.21s system 177% cpu 10:32.29 total**
* after_fastdebug: **1137.89s user 58.07s system 783% cpu 2:32.61 total**

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Commit messages:
 - Backport edd454b502b9bacde55492820e52655bbac63b89

Changes: https://git.openjdk.org/jdk11u-dev/pull/2155/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2155&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/jdk11u-dev/pull/2155.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2155/head:pull/2155

PR: https://git.openjdk.org/jdk11u-dev/pull/2155