libstdc++: Fix UB in weekday::weekday(sys_days) and add test

The following has undefined behaviour (signed overflow) [1]:
    weekday max{sys_days{days{numeric_limits<days::rep>::max()}}};

The issue is in this line when __n is very large and __n + 4 overflows:
    return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);

In addition to fixing this bug, the new implementation makes the compiler emit
shorter and branchless code for x86-64 and ARM [2].

[1] https://godbolt.org/z/1s5bv7KfT
[2] https://godbolt.org/z/zKsabzrhs

libstdc++-v3/ChangeLog:

	* include/std/chrono (weekday::_S_from_days): Fix UB.
	* testsuite/std/time/weekday/1.cc: Add test for overflow.