Use volatile on x86/x64 (native atomic guarantees) and C11 atomics on ARM (explicit synchronization needed). Restores performance on x86 while keeping ARM stability.

Summary

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Checklist

• Closes #xxx
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/

References

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