libstdc++: Populate generic std::time_get's wide %c format [PR117135]

libstdc++: Populate generic std::time_get's wide %c format [PR117135]

I missed out the __timepunct<wchar_t> specialization for the "generic"
implementation when defining the %c format in r15-4016-gc534e37faccf48.

libstdc++-v3/ChangeLog:

	PR libstdc++/117135
	* config/locale/generic/time_members.cc
	(__timepunct<wchar_t>::_M_initialize_timepunc): Set
	_M_date_time_format for C locale. Set %Ex formats to the same
	values as the %x formats.

(cherry picked from commit 707d84efee7f7eb5a336935f386e094402f267a6)