lib/date: Make rtc_mktime and mktime64 Y2038-ready

We currently overflow due to wrong types used internally in rtc_mktime,
on all platforms, and we return a too small type on 32-bit.

One consumer that directly benefits from this is mktime64. Many others
may still store the result in a wrong type.

While at it, drop the redundant cast of mon in rtc_mktime (obsoleted by
71420983).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>