libstdc++: Use correct duration for atomic_futex wait on custom clock [PR 91486]

As Jonathan Wakely pointed out[1], my change in commit
f9ddb696a289cc48d24d3d23c0b324cb88de9573 should have been rounding to
the target clock duration type rather than the input clock duration type
in __atomic_futex_unsigned::_M_load_when_equal_until just as (e.g.)
condition_variable does.

As well as fixing this, let's create a rather contrived test that fails
with the previous code, but unfortunately only when run on a machine
with an uptime of over 208.5 days, and even then not always.

[1] https://gcc.gnu.org/pipermail/libstdc++/2020-September/051004.html

libstdc++-v3/ChangeLog:

	PR libstdc++/91486
	* include/bits/atomic_futex.h:
	(__atomic_futex_unsigned::_M_load_when_equal_until): Use target
	clock duration type when rounding.
	* testsuite/30_threads/async/async.cc (test_pr91486_wait_for):
	Rename from test_pr91486.
	(float_steady_clock): New class for test.
	(test_pr91486_wait_until): New test.

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