MINOR: wdt: move wd_timer to wdt.c

MINOR: wdt: move wd_timer to wdt.c

The watchdog timer had no more reason for being shared with the struct
thread_info since the watchdog is the only user now. Let's remove it
from the struct and move it to a static array in wdt.c. This removes
some ifdefs and the need for the ugly mapping to empty_t that might be
subject to a cast to a long when compared to TIMER_INVALID. Now timer_t
is not known outside of wdt.c and clock.c anymore.