The ADC has an analog watchdog, you can use it in two ways for this purpose.
Measure the internal Vrefint and trigger an interrupt when it becomes too high, meaning that VDDA has started dropping.
You would have more time to react if you can measure the input of the external voltage regulator (if there is one) through a voltage divider (if the input voltage can be > 4 V) and an ADC pin, set the analog watchdog to interrupt when it is too low.
