You are misunderstanding what Tiers are. Embedded architectures cannot be Tier 1 which requires running Rust compiler compiled on the architecture itself and testing stuff on with it. Only full desktop systems will be Tier 1, because those are the systems that you can run Rustc and all the nice desktop environments.
However most of the embedded world uses ARM chips and they are Tier 2 like thumbv6m and thumbv7em (there are still odd ones like 8051 or AVR or m68k, many of them lack a good C++ compiler already). They are guaranteed to be built and at the release time the tests still run for them.
I understand your tiers just fine. You are misunderstanding what "cross platform" means. Or rather, you're trying to redefine it to mean "what Rust supports, in the way we want to support it, on the few architectures we care about, because in our view nothing else of value exists".
However most of the embedded world uses ARM chips and they are Tier 2 like thumbv6m and thumbv7em (there are still odd ones like 8051 or AVR or m68k, many of them lack a good C++ compiler already). They are guaranteed to be built and at the release time the tests still run for them.