G5 Quad hangs early on 4.20.2 / 5.0-rc2+

Hi,

both the latest stable 4.20.2 and 5.0 rc2+ hang early on the G5 Quad.

Surely I'm not the first to run into this, but I couldn't find any
discussion or bug report. Sorry if you're already aware.

You can see it hang here (5.0 rc2+, 4.20.2 is nearly identical) until
the watchdog triggers a reboot:

https://i.imgur.com/UiCVRuG.jpg

If I had to make an uneducated guess, it seems to boot into the same
codepath twice (mpic was already initialized, then it starts again right
after smp bringup). Maybe on a second CPU?

To narrow it down a little, my last known good was 4.18.9