RFR: 8374685: ZGC: Convert zMarkStack to use Atomic<T>

[Stefan Karlsson]

On Tue, 27 Jan 2026 10:56:16 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> Convert usages of AtomicAccess to Atomic<T> in zMarkStack.
> 
> I kept the old `compare_exchange` instead of doing a `compare_set` followed by a reload. We could probably support either. We would need to be careful in the `pop` loop, not sure if we gain much in readability from that change.
> 
> Testing:
>  * GHA
>  * HotSpot GC tests with ZGC linux x64 and linux aarch64

Marked as reviewed by stefank (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/29446#pullrequestreview-3710551798