[jdk16u] RFR: 8261413: Shenandoah: Disable class-unloading in I-U mode

[Aleksey Shipilev]

This workarounds the bug in non-default Shenandoah mode. The risk is low, as it only affects Shenandoah code.

Additional testing:
 - [x] hotspot_gc_shenandoah

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Commit messages:
 - Backport e2d52ae2656deab98229a4701810334e9cc8519a

Changes: https://git.openjdk.java.net/jdk16u/pull/54/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=54&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261413
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/54.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/54/head:pull/54

PR: https://git.openjdk.java.net/jdk16u/pull/54