[jdk21u] RFR: 8319633: runtime/posixSig/TestPosixSig.java intermittent timeouts on UNIX

[Joachim Kern]

We should not mix os::dll_load() with dlclose(), but should call os::dll_unload(). At the moment this is benign, but this prevents certain type of platforms specific workarounds inside os::dll_load() and is a prerequisite for these upcoming changes.

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Commit messages:
 - Backport 464dc3dab540a36aeea76dbeea548c600df58763

Changes: https://git.openjdk.org/jdk21u/pull/407/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319633
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/407.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/407/head:pull/407

PR: https://git.openjdk.org/jdk21u/pull/407