[jdk17u-dev] RFR: 8319876: Reduce memory consumption of VM_ThreadDump::doit

[Elif Aslan]

This pull request contains a backport of commit [8ec6b8de3bb3d7aeebdcb45d761b18cce3bab75e](https://github.com/openjdk/jdk/commit/8ec6b8de3bb3d7aeebdcb45d761b18cce3bab75e) from the openjdk/jdk repository to JDK 17.

This backport significantly reduces RSS memory usage during ThreadMXBean.dumpAllThreads(boolean, boolean) execution.

This is not a clean backport due to differences between JDK 17 and upstream:

Threading Context: Changed ResourceMark rm(VMThread::vm_thread()) to ResourceMark rm;
   • Similar to [JDK 21 backport](https://github.com/openjdk/jdk21u-dev/pull/307)

Minor conflict resolved: _threads = nullptr; → _threads = NULL; to match JDK 17 conventions

GHA tested additionally ran tier1 tests on my local mac and passed.

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Commit messages:
 - backport of 8ec6b8de3bb3d7aeebdcb45d761b18cce3bab75e

Changes: https://git.openjdk.org/jdk17u-dev/pull/4006/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4006&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319876
  Stats: 3 lines in 2 files changed: 1 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/4006.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4006/head:pull/4006

PR: https://git.openjdk.org/jdk17u-dev/pull/4006