[libc++] Make __weekday_from_days private in weekday

`weekday` has a static member function `__weekday_from_days` which is
not part of the mandated public interface of `weeekday` according to the
standard. Since it is only used internally in the constructors of
`weekday`, let's make it private.

Reviewed By: ldionne, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D112072