RFR: 8364176: Serial: Group all class unloading logc at the end of marking phase

[Albert Mingkun Yang]

Simple moving some classunloading logic inside `SerialFullGC`.

Test: tier1

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Commit messages:
 - sgc-classunloading

Changes: https://git.openjdk.org/jdk/pull/26499/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26499&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364176
  Stats: 22 lines in 2 files changed: 9 ins; 10 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26499.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26499/head:pull/26499

PR: https://git.openjdk.org/jdk/pull/26499