RFR: JDK-8288003: log events for os::dll_unload

[Matthias Baesken]

Currently we only log events for os::dll_load, but not for os::dll_unload, this patch adds it. On some platforms (Linux/Windows) we can use OS APIs (e.g. dlinfo on Linux) to log the path of the unloaded shared lib.

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Commit messages:
 - JDK-8288003

Changes: https://git.openjdk.java.net/jdk/pull/9101/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288003
  Stats: 35 lines in 2 files changed: 33 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9101.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9101/head:pull/9101

PR: https://git.openjdk.java.net/jdk/pull/9101