There are just too many niches where the knowledge we each consider necessary simply isn't. I had one particularly bad interviewer grill me on how the ARM GIC worked in detail (e.g. interconnect details, differences between versions, etc) because they considered it basic knowledge. I've personally never needed to know anything about it that wasn't in a TRM.
One question I have found useful in embedded development is asking someone to discuss the difference between a thread and a process, and the difference between thread based OSs and process based OSs. It is a general question, not bound by anything like CPU architecture, but just gives an idea into whether the person is comfortable about general memory domains.
I have mentored people, bright programmers that never worked in small embedded systems, that initially tripped all over the thread model, but eventually came to understand it.