libstdc++: More efficient days from date

This patch reimplements std::chrono::year_month_day::_M_days_since_epoch()
which calculates the number of elapsed days since 1970/01/01.  The new
implementation is based on Proposition 6.2 of Neri and Schneider, "Euclidean
Affine Functions and Applications to Calendar Algorithms" available at
https://arxiv.org/abs/2102.06959.

The aforementioned paper benchmarks the implementation against several
counterparts, including libc++'s (which is identical to the current
implementation).  The results, shown in Figure 3, indicate the new algorithm is
1.7 times faster than the current one.

The patch adds a test which loops through all dates in [-32767/01/01,
32767/12/31], and for each of them, gets the number of days and compares the
result against its expected value. The latter is calculated using a much
simpler and easy to understand algorithm but which is also much slower.

The dates used in the test covers the full range of possible values
[time.cal.year.members].  Despite its completeness the test runs in matter of
seconds.

libstdc++-v3/ChangeLog:

	* include/std/chrono (year_month_day::_M_days_since_epoch):
	New implementation.
	* testsuite/std/time/year_month_day/4.cc: New test.

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