RFR: 8367717: Cleanup atomic_copy64

[jonghoonpark]

related jira issue: https://bugs.openjdk.org/browse/JDK-8367717

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

## Verification
- Verified by running: `make run-test TEST="gtest:AtomicAccess*"` with a JDK image built using the `zero` JVM variant.

Please let me know if there are any additional or more appropriate verification steps I should perform.

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Commit messages:
 - cleanup: align atomic_copy64 definitions for zero platforms with others

Changes: https://git.openjdk.org/jdk/pull/27642/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27642&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367717
  Stats: 28 lines in 4 files changed: 14 ins; 14 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27642.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27642/head:pull/27642

PR: https://git.openjdk.org/jdk/pull/27642