Not necessarily. C isn't a great language to target, since it's full of fun undefined behavior quirks that you will trip over if you look at it slightly cross-eyed. And part of the problem with the long tail platforms is that they can have issues that you don't expect (e.g., function pointers and data pointers might have different representation!).

So the main set of platforms you're looking at are ones that are functional enough to have modern, stable C compilers, aren't too quirky that reasonably portable C code would "just" work, and yet ones that you don't already have support for in Rust. Outside of the embedded space, there's like... fewer than 5 architectures left, the most notable of which probably being Alpha.