Keep bthread TaskGroup abi compatible with NDEBUG macro

[wwbmmm]

What problem does this PR solve?

Issue Number: #2042

Problem Summary: bthread::TaskGroup add a new member _sched_recursive_guard if NDEBUG is not defined, this cause abi incompatible.

What is changed and the side effects?

Changed: define the _sched_recursive_guard whether NDEBUG is defined or not

Side effects:

• Performance effects(性能影响): No

• Breaking backward compatibility(向后兼容性): No

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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).
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