RFR: 8364516: Serial: Move class unloading logic inside SerialFullGC::invoke_at_safepoint

[Albert Mingkun Yang]

On Fri, 1 Aug 2025 08:47:57 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> 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

Thank you for the feedback; applied all except this one -- "the other" phases probably don't make much sense, because this comment precedes all full-gc phases; there is no "current" phase.

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PR Comment: https://git.openjdk.org/jdk/pull/26588#issuecomment-3143931687