sys/arduino: use 64bit usec to compute millis()

This is quick solution to avoid wrapping around after 4294967 milliseconds.
It uses xtimer_now_usec64 instead of xtimer_now_usec.

Notice that this is more expansive than the previous solution, especially
on AVR systems.

sys/arduino/base.cpp

@@ -74,7 +74,7 @@ unsigned long micros()
 
 unsigned long millis()
 {
-    return xtimer_now_usec() / US_PER_MS;
+    return xtimer_now_usec64() / US_PER_MS;
 }
 
 int analogRead(int arduino_pin)