RFR: 8315364: Assert thread state invariant for JFR stack trace capture

[Markus Grönlund]

Greetings,

A JavaThread running in state _thread_in_native cannot capture a JFR stack trace because tagging of artefacts (klasses, methods, etc.) is a function of an epoch, which evolves during safepoints.

Asserting this invariant would have helped prevent the issue detailed in [JDK-8315220](https://bugs.openjdk.org/browse/JDK-8315220).

Testing: jdk_jfr

Thanks
Markus

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Commit messages:
 - 8315364

Changes: https://git.openjdk.org/jdk/pull/15491/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315364
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15491.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15491/head:pull/15491

PR: https://git.openjdk.org/jdk/pull/15491