RFR: 8360201: JFR: Initialize JfrThreadLocal::_sampling_critical_section [v2]

[Aleksey Shipilev]

> Initially found by UBSan. I was not able to reproduce it locally, @MBaesken would try. But the problem seems obvious: the initial value for the `bool` field is garbage. I have checked other fields in `JfrThreadLocal`, they seem fine.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8360201-jfr-init-threadlocal
 - Fix

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25938/files
  - new: https://git.openjdk.org/jdk/pull/25938/files/e2419224..2c845b09

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25938&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25938&range=00-01

  Stats: 1492 lines in 92 files changed: 189 ins; 239 del; 1064 mod
  Patch: https://git.openjdk.org/jdk/pull/25938.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25938/head:pull/25938

PR: https://git.openjdk.org/jdk/pull/25938