RFR: 8364516: Serial: Move class unloading logic inside SerialFullGC::invoke_at_safepoint [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into sgc-unloading-doc
 - review
 - review
 - review
 - sgc-unloading-doc

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26588/files
  - new: https://git.openjdk.org/jdk/pull/26588/files/88fe17c7..a912850a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26588&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26588&range=02-03

  Stats: 2991 lines in 75 files changed: 2663 ins; 119 del; 209 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