[Bug libstdc++/58938] std::exception_ptr is missing on architectures with incomplete atomic int support

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58938

--- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> ---
A prototype implementation that doesn't rely on atomics:

https://gcc.gnu.org/ml/gcc-patches/2016-09/msg01526.html
https://gcc.gnu.org/ml/gcc-patches/2016-09/msg01546.html
(both patches are needed).