libstdc++: Improve operator-(weekday x, weekday y)

The current implementation calls __detail::__modulo which is relatively
expensive.

A better implementation is possible if we assume that x.ok() && y.ok() == true,
so that n = x.c_encoding() - y.c_encoding() is in [-6, 6]. In this case, it
suffices to return n >= 0 ? n : n + 7.

The above is allowed by [time.cal.wd.nonmembers]/5: the returned value is
unspecified when x.ok() || y.ok() == false.

The assembly emitted for x86-64 and ARM can be seen in:
https://godbolt.org/z/nMdc5vv9n.

libstdc++-v3/ChangeLog:

	* include/std/chrono (operator-(const weekday&, const weekday&)):
	Optimize.