I kid. The entire Zen class of AMD processors are vuln to Inception so you're not safe anywhere.
There is the "just use OS/2" strategy of the 1990s (referring to the period from 1995-2002 where you could sit happy in your niche OS that researchers and BHs were not targeting): in this case s390x probably has the same problem but no one is paying attention to it (other than a 3 letter agency, of course), so the chances of a payload reaching that arch are very close to zero.
There is the "just use OS/2" strategy of the 1990s (referring to the period from 1995-2002 where you could sit happy in your niche OS that researchers and BHs were not targeting): in this case s390x probably has the same problem but no one is paying attention to it (other than a 3 letter agency, of course), so the chances of a payload reaching that arch are very close to zero.