Depends what you want to compare (CPU horsepower, volatile storage, non-volatile storage, I/O bandwidth, something else). I really don’t know but my guess is that it would be so far behind the Librem 5 (or any other contemporary smartphone) that no comparison is worthwhile. https://www.allaboutcircuits.com/news/voyager-mission-anniversary-computers-command-data-attitude-control/
Edit: See also What kind of computers are used on the Voyager spacecraft? in Voyager - Frequently Asked Questions