fix gdb bthread stack lookup symbol error when using libc++

[ehds]

What problem does this PR solve?

Issue Number:

Problem Summary:
当 brpc 使用 libc++ 编译时，使用当前 gdb bthread 调试脚本会出现找不到符号错误（ _M_i, ._M_b._M_p 等变量），这些变量应该是来自 libstdc++ 实现 std::atomic 的中的，而当使用 libc++ 时没有这些变量，所以会出现此类错误。

(gdb) bthread_begin 
Python Exception <class 'gdb.error'>: There is no member or method named _M_i.
Error occurred in Python: There is no member or method named _M_i.

What is changed and the side effects?

Changed:

使用类型强转实现从 atomic<T> -> T 的转化，避免使用特定标准库实现中的变量。

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).