RFR: 8369420: Parallel: Using multiple workers for class unloading

[Albert Mingkun Yang]

Support parallel class unloading in ParallelGC. Much inspiration taken from `G1ParallelCleaningTask`.

Test: tier1-5

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Commit messages:
 - pgc-class-unloading

Changes: https://git.openjdk.org/jdk/pull/27689/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27689&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369420
  Stats: 45 lines in 1 file changed: 34 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27689.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27689/head:pull/27689

PR: https://git.openjdk.org/jdk/pull/27689