[v3] mutex vs. cstdatomic

[Benjamin Kosnik]

> With a straight up swap of bool for atomic_bool, I'm seeing a lot of
> these:
> 
> atomic_2.h:312: _ITp
> std::__atomic2::__atomic_base<_IntTp>::load(std::memory_order) const
> volatile [with _ITp = bool]:
>  Assertion `__m == memory_order_release' failed.

This was intended, as memory_order_release doesn't meet the
specified requirements for this function. 

-benjamin