The earliest Intel Atom chips (Nxxx series) are supposedly safe, but they were only ever used in woefully underpowered netbooks and nettops, and they had perhaps half the performance of a similarly clocked (much older) Pentium M. That performance metric is documented, and I've felt it myself when I owned a Pentium M laptop and Atom N450 netbook at the same time a few years ago.

A few ARM SoCs -- including the entire line used in Raspberry Pi boards -- are safe, but the vast majority of recent ARM devices are affected by one or more of the attack vectors. This means virtually any flagship and most if not all midrange smartphones and tablets, even iPhones and iPads, are vulnerable.

This is the most complete list of affected CPUs and SoCs I've found, and they appear to be keeping it updated:

https://www.techarp.com/guides/complete-meltdown-spectre-cpu...

I think it's safe to assume that pratical mitigations will eventually surface, the biggest issue is probably around the cost in performance. Shaving 30% (or whatever) of the worlds computing power in one fell swoop is kind of a big deal.

Arm (starting with Cortex-R7 and higher) and PowerPC are vulnerable to Spectre too.