[Bug libstdc++/93542] std::future::wait_for should use monotonic clock

[mac at mcrowe dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93542

--- Comment #2 from Mike Crowe <mac at mcrowe dot com> ---
An updated version of the fix is available in
https://gcc.gnu.org/pipermail/libstdc++/2020-May/050433.html which relies on
https://gcc.gnu.org/pipermail/libstdc++/2020-May/050439.html .