Fix stale AgentGroup TLS access after bthread migration

[bobhan1]

What problem does this PR solve?

Issue Number: resolve #3520

Problem Summary:

bvar::detail::AgentGroup directly accesses the raw _s_tls_blocks thread-local variable from inline functions. When a bthread suspends and resumes on another pthread, Clang may reuse the TLS address resolved for the previous pthread. This can make bvar access another pthread's agent vector and race with its initialization or resize.

What is changed and the side effects?

Changed:

• Declare _s_tls_blocks with the existing static-member volatile TLS accessor mechanism.
• Resolve the current pthread's TLS block vector once in each AgentGroup operation and consistently use that pointer.
• Route TLS initialization and cleanup writes through the existing setter accessor.

Side effects:

• Performance effects: On compiler targets where volatile TLS access is enabled, AgentGroup operations use the existing noinline TLS accessor to prevent the TLS address from being retained across a bthread suspend point. Other targets retain direct TLS access through the existing macros.
• Breaking backward compatibility: None.

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Check List:

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