RFR: 8364516: Serial: Move class unloading logic inside SerialFullGC::invoke_at_safepoint [v3]

[Albert Mingkun Yang]

> Encapsulating classunloading logic inside `invoke_at_safepoint` and adding doc explaining why the classunloading logic can't be placed at the end of marking during full-gc, like other STW collectors.
> 
> Test: tier1-3

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26588/files
  - new: https://git.openjdk.org/jdk/pull/26588/files/f41fb5ef..88fe17c7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26588&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26588&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26588.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26588/head:pull/26588

PR: https://git.openjdk.org/jdk/pull/26588