[Bug libstdc++/56538] no way to disable shared_ptr's use of atomics

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56538

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We also support __shared_ptr<T, _S_single> which avoids the atomic dispatching
code entirely.