Use explicit C11 atomic wrappers with relaxed defaults

[devreal]

Summary

• Wrap C11 _Atomic types in structs to prevent direct access and define memory order enums
• Add explicit load/store helpers with optional memory order argument, defaulting to relaxed
• Update code to use new opal_atomic_load/store APIs and initialize atomics via OPAL_ATOMIC_INIT

Testing

• ./autogen.pl (fails: could not find a recent enough copy of libtool)

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https://chatgpt.com/codex/tasks/task_b_68a372de25408326b0cd9e6f9e0147a4

[github-actions]

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cb19d6a: Use struct-wrapped C11 atomics with explicit order...

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