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

[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

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Depends on: https://git.openjdk.org/jdk/pull/24296

Commit messages:
 - 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/71c284ae...d180ce01

Changes: https://git.openjdk.org/jdk/pull/25302/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25302&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337789
  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