posix: implement `timer_getoverrun()`

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, timer_getoverrun() is one of them.

timer_getoverrun, timer_gettime, timer_settime - per-process timers

The timer_gettime() function shall store the amount of time until the specified timer, timerid, expires and the reload value of the timer into the space pointed to by the value argument. The it_value member of this structure shall contain the amount of time before the timer expires, or zero if the timer is disarmed. This value is returned as the interval until timer expiration, even if the timer was armed with absolute time. The it_interval member of value shall contain the reload value last set by timer_settime().
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See https://pubs.opengroup.org/onlinepubs/9699919799/functions/timer_getoverrun.html

See RFC #51211 for more info.