RFR: 8319876: Reduce memory consumption of VM_ThreadDump::doit

[Long Yang]

I would like to fix this.

Create 1000 threads, and the stack depth of each thread is 1000. 
After running jmx.dumpAllThreads(true, true), the RSS reaches 5.2GiB. 
After optimization, the RSS is 250MiB.

I would appreciate it if anyone could review this.

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Commit messages:
 - 8319876: Reduce memory consumption of VM_ThreadDump::doit

Changes: https://git.openjdk.org/jdk/pull/16598/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16598&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/jdk/pull/16598.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16598/head:pull/16598

PR: https://git.openjdk.org/jdk/pull/16598