Since Nim uses C as an intermediate representation, cross-compilation support should be as good as your C compiler's.
Rust's cross-compilation seems to not be that great at the moment. Although i believe that's because it hasn't been done yet, rather than having been done badly.
Rust's cross-compilation works, but it's not as easy as it could be. One issue is that you need a cross-compiled version of the standard library lying around.
http://nim-lang.org/nimc.html#cross-compilation
Since Nim uses C as an intermediate representation, cross-compilation support should be as good as your C compiler's.
Rust's cross-compilation seems to not be that great at the moment. Although i believe that's because it hasn't been done yet, rather than having been done badly.