[caf]

...how closely tied it is to the x86 memory model & the PC in general.

Is it, though? Linux has had NOMMU (running without an MMU at all) support since the early 2000s, and the atomics / barriers are more based on Alpha's memory model than anything else - several of those primitives just compile out to a compiler barrier or nothing at all on x86.

Most heavyweight things you don't need in an IoT context can be compiled-out completely - I think probably the major bit of infrastructure you can't is support for multiple user IDs?