RFR: 8280450: Add task queue printing to STW Full GCs

[Thomas Schatzl]

Hi all,

  please review this change that adds task queue statistics printing after full gc marking. This helps diagnosing performance issues there.

Testing: compilation, gha, local testing (all new code is guarded by `TASKQUEUE_STATS` so not compiled in by default)

Thanks,
  Thomas

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/7264/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7264&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280450
  Stats: 8 lines in 2 files changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7264.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7264/head:pull/7264

PR: https://git.openjdk.java.net/jdk/pull/7264