Not OP, but just a set of MCU arch common in automotive that you'd have to bend a bit to compile Rust for:
- Renesas RH 850/v850 - there is a gcc but not a llvm backend
- RL78 (I think this is kind of dieing though)
- Infineon Aurix Tricore (Hightec has a compiler for it)
- PowerPC was pretty popular but I did not see it that much lately.