libstdc++: Fix std::format output of %C for negative years

During discussion of LWG 4022 I noticed that we do not correctly
implement floored division for the century. We were just truncating
towards zero, rather than applying the floor function. For negative
values that rounds the wrong way.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_C_y_Y): Fix
	rounding for negative centuries.
	* testsuite/std/time/year/io.cc: Check %C for negative years.