Re: Not sure what they used...
I think the only rad-hard CPU around at the time was the RCA 1802:
A completely static design with no minimum clock frequency and you could use any register for the program counter.
The Z80 wouldn't have been released in time for this mission and certainly wasn't rad-hard.
Edit: I've just read the Wiki page and even the 1802 wasn't around in time for the Voyager. Also read: