[Bug libstdc++/78237] std::timed_mutex::try_lock_for/until affected by system realtime clock

[mac at mcrowe dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78237

--- Comment #3 from Mike Crowe <mac at mcrowe dot com> ---
Proposed fix for std::timed_mutex and std::shared_timed_mutex in series posted
for review at https://gcc.gnu.org/ml/gcc-patches/2019-09/msg01646.html .