libstdc++: Avoid rounding errors in std::future::wait_* [PR 91486]

libstdc++: Avoid rounding errors in std::future::wait_* [PR 91486]

Convert the specified duration to the target clock's duration type
before adding it to the current time in
__atomic_futex_unsigned::_M_load_when_equal_for and
_M_load_when_equal_until.  This removes the risk of the timeout being
rounded down to the current time resulting in there being no wait at all
when the duration type lacks sufficient precision to hold the
steady_clock current time.

Rather than using the style of fix from PR68519, let's expose the C++17
std::chrono::ceil function as std::chrono::__detail::ceil so that it can
be used in code compiled with earlier standards versions and simplify
the fix. This was suggested by John Salmon in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91486#c5 .

This problem has become considerably less likely to trigger since I
switched the __atomic__futex_unsigned::__clock_t reference clock from
system_clock to steady_clock and added the loop, but the consequences of
triggering it have changed too.

By my calculations it takes just over 194 days from the epoch for the
current time not to be representable in a float. This means that
system_clock is always subject to the problem (with the standard 1970
epoch) whereas steady_clock with float duration only runs out of
resolution machine has been running for that long (assuming the Linux
implementation of CLOCK_MONOTONIC.)

The recently-added loop in
__atomic_futex_unsigned::_M_load_when_equal_until turns this scenario
into a busy wait.

Unfortunately the combination of both of these things means that it's
not possible to write a test case for this occurring in
_M_load_when_equal_until as it stands.

libstdc++-v3/ChangeLog:

	PR libstdc++/91486
	* include/bits/atomic_futex.h
	(__atomic_futex_unsigned::_M_load_when_equal_for)
	(__atomic_futex_unsigned::_M_load_when_equal_until): Use
	__detail::ceil to convert delta to the reference clock
	duration type to avoid resolution problems.
	* include/std/chrono (__detail::ceil): Move implementation
	of std::chrono::ceil into private namespace so that it's
	available to pre-C++17 code.
	* testsuite/30_threads/async/async.cc (test_pr91486):
	Test __atomic_futex_unsigned::_M_load_when_equal_for.