Fix 32-bit overflow in mktime() when time_t is 64-bits long

When converting number of days since epoch (32-bits) to seconds,
calculations using 32-bit `long` overflow for years above 2038. Solve
this by casting number of days to `time_t` just before final
multiplication.

Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>

newlib/libc/time/mktime.c

@@ -188,7 +188,7 @@ mktime (struct tm *tim_p)
     }
 
   /* compute total seconds */
-  tim += (days * _SEC_IN_DAY);
+  tim += (time_t)days * _SEC_IN_DAY;
 
   TZ_LOCK;