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
714209832d).

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

include/linux/time.h

@@ -152,9 +152,6 @@ _DEFUN (ctime_r, (tim_p, result),
     return asctime_r (localtime_r (tim_p, &tm), result);
 }
 
-/* for compatibility with linux code */
-typedef __s64 time64_t;
-
 #ifdef CONFIG_LIB_DATE
 time64_t mktime64(const unsigned int year, const unsigned int mon,
 		  const unsigned int day, const unsigned int hour,