Having a Rust compiler is probably unavoidable

I read that as saying it's simpler to port GCC or LLVM to a new platform because you probably want more than a C compiler (that you could in theory write yourself) you want C++ and other languages which GCC and LLVM support, that would be prohibitively expensive to write a new independent implementation for on your new platform. If the path of least resistance is already porting GCC or LLVM and Rust can be compiled using LLVM and soon GCC then your platform can also get Rust support because much of the underlying work will have already been done, it's not as if anyone needs to write a new Rust compiler for your new platform.