[jdk23] RFR: 8342905: Thread.setContextClassloader from thread in FJP commonPool task no longer works after JDK-8327501 redux

[Viktor Klang]

Having opted in to allow setting an SM is a clearer signal.

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Commit messages:
 - Better fix for Thread.setContextClassloader from thread in FJP commonPool task no longer works after JDK-8327501

Changes: https://git.openjdk.org/jdk/pull/21668/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21668&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342905
  Stats: 7 lines in 1 file changed: 4 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21668.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21668/head:pull/21668

PR: https://git.openjdk.org/jdk/pull/21668