[jdk11u-dev] RFR: 8215889: assert(!_unloading) failed: This oop is not available to unloading class loader data with ZGC

[Martin Doerr]

Backport of JDK-8215889. Applies cleanly, but the change needs a small adaptation because safepoint counter uses `int` instead of `uint64_t` in 11u (JDK-8212108 is not in 11u).

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Commit messages:
 - Backport f08eeac278b0785c0bdff0bddf46b66f65822357

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/434/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=434&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8215889
  Stats: 117 lines in 8 files changed: 103 ins; 10 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/434.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/434/head:pull/434

PR: https://git.openjdk.java.net/jdk11u-dev/pull/434