Not all boards that provide a Arduino pin layout break out all GPIOs. A good example are Adafruit `feather-m0` boards. GPIOs that are not broken out have to be defined as `GPIO_UNDEF` to preserve the Arduino pin layout. However, GPIO functions lead to a complete system lock on `feather-m0` boards if a pin is used that is defined as `GPIO_UNDEF`. Therefore, at least an assert should blow up in this case.
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.
