Check out the Clang extended vector extension for C, IMHO that's the perfect way to add portable SIMD to a language. Doing it through a library will always be clumsy.
But a "new" language like Rust would have the freedom to add special language syntax and operators for such things instead of going the C++ way of "just add it to the stdlib" (which IMHO is almost always the wrong place... it should either go into a regular cargo dependency, or into the language itself).