[Bug libstdc++/68921] [5/6 Regression] std::future::wait() makes invalid futex calls and spins

[torvald at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68921

--- Comment #3 from torvald at gcc dot gnu.org ---
LGTM, thanks.  Would be nice to backport this.