RFR: 8337789: JEP 509: JFR CPU-Time Profiling (Experimental) [v2]

[Johannes Bechberger]

> This is the code for the [JEP 509: CPU Time based profiling for JFR](https://openjdk.org/jeps/509).
> 
> Currently tested using [this test suite](https://github.com/parttimenerd/basic-profiler-tests). This runs profiles the [Renaissance](https://renaissance.dev/) benchmark with
> - ... different heap sizes
> - ... different GCs
> - ... different samplers (the standard JFR and the new CPU Time Sampler and both)
> - ... different JFR recording durations
> - ... different chunk-sizes

Johannes Bechberger has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:

 - Remove unused WhiteBox method
 - Read _active_signal_handlers atomically
 - Check whether raw_thread is not null and is JavaThread is signal handler
 - Tiny fixes
 - Fix uninitialized variable
 - Reduce diff
 - Refactor
 - Reduce diff
 - Reduce diff
 - Tiny fixes
 - ... and 11 more: https://git.openjdk.org/jdk/compare/46566a9b...24216fe8

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Changes: https://git.openjdk.org/jdk/pull/25302/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25302&range=01
  Stats: 2335 lines in 46 files changed: 2184 ins; 130 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/25302.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25302/head:pull/25302

PR: https://git.openjdk.org/jdk/pull/25302