[committed] libstdc++: Optimise std::future::wait_for and fix futex polling

[Jonathan Wakely]

>Backporting the change to gcc-10 revealed an overflow bug in the
>existing code, resulting in blocking for years when given an absolute
>timeout in the distant past. There's still a similar bug in the new
>code (using futexes with absolute timeouts against clocks) where a
>large chrono::seconds value can overflow and produce an incorrect
>tv_sec value. Apart from the overflow itself being UB, the result in
>that case is just a spurious wakeup (the call says it timed out when
>it didn't reach the specified time). That should still be fixed, but
>I'll do it separately.

And here's the separate fix for that other overflow. In fact it
doesn't just produce spurious wakeups. It either spins forever
(instead of sleeping gently) or incorrectly reports a timeout when it
should keep blocking. Either way, it's fixed now.

Tested x86_64-linux && powerpc64le-linux. Committed to trunk.


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