[jdk23u] RFR: 8342905: Thread.setContextClassloader from thread in FJP commonPool task no longer works after JDK-8327501 redux

[Viktor Klang]

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/jdk23u/pull/207/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk23u&pr=207&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/jdk23u/pull/207.diff
  Fetch: git fetch https://git.openjdk.org/jdk23u.git pull/207/head:pull/207

PR: https://git.openjdk.org/jdk23u/pull/207