validate target userspace architecture matches kernel's architecture

[bradfitz]

For gokrazy/gokrazy#191.

Perhaps we'll make it automatic in the future, though. This is a guardrail meanwhile.

[stapelberg]

Thanks for sending this :) I have made this mistake myself, and I know of a few others, too…

[damdo]

This is gold, I spent two days debugging an issue with the arch once. Thanks!