The rustc compiler uses LLVM, so whatever new architectures LLVM supports, rustc also gets. There are two projects going on right now - rustc_codegen_gcc which

The rustc compiler uses LLVM, so whatever new architectures LLVM supports, rustc also gets. There are two projects going on right now - rustc_codegen_gcc which adds GCC as a rustc backend, and gcc-rs which adds a rust frontend to GCC. Both have seen considerable progress and support over the past year and are probably the most promising avenue to expanding the architectures that rust can compile to.