RFR: 8361404: Parallel: Group all class unloading logc at the end of marking phase

[Albert Mingkun Yang]

Simple patch of moving some out-of-place `ClassLoaderDataGraph` logic to the end of marking-phase in order group all class unloading logic at the same scope/site.

Test: tier1-8

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Commit messages:
 - pgc-unloading

Changes: https://git.openjdk.org/jdk/pull/26130/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26130&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361404
  Stats: 28 lines in 1 file changed: 11 ins; 14 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26130.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26130/head:pull/26130

PR: https://git.openjdk.org/jdk/pull/26130