Rust actually supports most architectures(SPARC, AMD64/x86-64, ARM, PowerPC, RISC-V, and AMDGPU*). The limitations are from LLVM not supporting some architects(Alpha, SuperH, and VAX) and some instruction sets(sse2, etc.); z/Architecture is a bit of an outlier that has major challenges to overcome for LLVM-Rust. This is not going to be a problem when GCC-Rust is finished.
AMDGPU, *Not 100%, but works well enough to actually use in production and gets better all the time.