AMD has just alerted us that they have released a BIOS fix to their motherboard partners that takes care of the issue around booting newer Linux distributions on the new Zen 2 processors.
Earlier this week I mentioned AMD would be working on a BIOS fix to address the fundamental problem with booting newer systemd-using Linux distributions on their new Ryzen 3000 series processors. However, I hadn't expected the fix to make it to motherboard vendors in less than one week!
The problem is the RdRand issue colliding with systemd that is making use of the RdRand instruction directly and not jiving with the expected behavior. There's been a patch in systemd since May but that hasn't been found in a released version yet. But for newer Linux distributions like Ubuntu 19.04, Clear Linux, Fedora Workstation 31, Arch Linux / Manjaro, and others, it's meant not being able to boot the distribution due to all systemd services failing to start.