[jdk21u] RFR: 8315362: NMT: summary diff reports threads count incorrectly

[Evgeny Ignatenko]

Almost clean backport of [JDK-8315362](https://bugs.openjdk.org/browse/JDK-8315362)

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Commit messages:
 - Backport 37aed6f46d19a48b7bceffde61b529a69a6abf54

Changes: https://git.openjdk.org/jdk21u/pull/310/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=310&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315362
  Stats: 86 lines in 5 files changed: 73 ins; 11 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u/pull/310.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/310/head:pull/310

PR: https://git.openjdk.org/jdk21u/pull/310