libstdc++: Fix regression in std::format output of %Y for negative years

The change in r14-6468-ga01462ae8bafa8 was only supposed to apply to %C
formats, not %Y.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_C_y_Y): Do
	not round century down for %Y formats.

(cherry picked from commit ad537ccd)