fix: add acquire fence in bthread_join for ARM memory visibility

[guoliushui]

Ensure memory visibility on ARM architecture after thread join.

What problem does this PR solve?

Issue Number: fix #3274

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[guoliushui]

已使用测试程序验证生效

[Copilot]

Pull request overview

This PR addresses weak-memory-ordering visibility issues where bthread_join() may return before the joining thread can reliably observe the joined bthread’s prior memory writes (reported on ARM/aarch64).

Changes:

• Add an acquire fence after TaskGroup::join() observes the joined bthread has advanced version_butex.
• Gate the fence with ARM architecture preprocessor checks (__aarch64__ / __arm__).

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[guoliushui]

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