[jdk11u-dev] RFR: 8322752: GetStackTraceAndRetransformTest.java is failing assert

[Jaroslav Bachorik]

This pull request is addressing an omission in the backport of [JDK-8313816](https://bugs.openjdk.org/browse/JDK-8313816) where a new WhiteBox method to induce metaspace cleanup should have been invoked as a VM_Op to make sure the cleanup is happening in a safepoint.

Failing to do so will trigger assert when the test run is run against a debug build of JDK.

The fix is straightforward, creating a new `VM_WhiteBoxCleanMetaspaces` VM operation and then using it to execute `ClassLoaderDataGraph::do_unloading`.

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Commit messages:
 - 8322752: GetStackTraceAndRetransformTest.java is failing assert

Changes: https://git.openjdk.org/jdk11u-dev/pull/2423/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2423&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322752
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2423.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2423/head:pull/2423

PR: https://git.openjdk.org/jdk11u-dev/pull/2423