datetime: add fallback logic for AM/PM specifier
datetime: add fallback logic for AM/PM specifier

When the locale does not provide any string for AM/PM, fallback to the
default.

https://bugzilla.gnome.org/show_bug.cgi?id=761889

johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib