RFR: 8367717: Cleanup atomic_copy64

[Kim Barrett]

On Mon, 6 Oct 2025 04:58:16 GMT, jonghoonpark <duke at openjdk.org> wrote:

> As described in the issue, this change aligns atomic_copy64 definitions for
> zero platforms with those of other platforms.

The issue suggests it might be better to replace calls to atomic_copy64 with
`AtomicAccess::store(dst, AtomicAccess::load(src))` (using the new
`AtomicAccess` nomenclature), and then delete all definitions of
atomic_copy64. And there's a comment agreeing with that suggestion. I think
that's the way to go, rather than what this PR proposes.

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Changes requested by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27642#pullrequestreview-3303009902