RFR: 8370774: Merge ModRefBarrierSet into CardTableBarrierSet

[Thomas Schatzl]

On Tue, 28 Oct 2025 09:20:46 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Merge a class into its sole subclass.
> 
> Many files are changed in this PR, and they can be largely divided into two groups, moving content of `ModRefBarrierSet` into `CardTableBarrierSet` (`src/hotspot/share/gc/`) and platform specific moving content of `ModRefBarrierSetAssembler` into `CardTableBarrierSetAssembler` (`src/hotspot/cpu/`).
> 
> Test: tier1-5 for x64 and aarch64; GHA

Seems good.

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Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28013#pullrequestreview-3402999525