best_effort_wfe_or_timeout() function

Helper method for blocking on a timeout This method will return in response to an event (as per __wfe) or when the target time is reached, or at any point before. This method can be used to implement a lower power polling loop waiting on some condition signalled by an event (__sev()). This is called \a best_effort because under certain circumstances (notably the default timer pool being disabled or full) the best effort is simply to return immediately without a __wfe, thus turning the calling code into a busy wait. Example usage: ```c bool my_function_with_timeout_us(uint64_t timeout_us) { absolute_time_t timeout_time = make_timeout_time_us(timeout_us); do { // each time round the loop, we check to see if the condition // we are waiting on has happened if (my_check_done()) { // do something return true; } // will try to sleep until timeout or the next processor event } while (!best_effort_wfe_or_timeout(timeout_time)); return false; // timed out } ```