Someone has to come in and make it supported. Adopting nlvm isn't going to magically make it easier.
The reason it's easier in Rust is because someone that's passionate about WASM came along and did the work up front to document everything and make it as simple to compile to WASM as possible, the same is certainly possible in Nim, just need someone to put in some work :)
I really do like Nim, but suggesting it to do WebAssembly out of the box could be deceiving and do no good. The project is currently in the hard spot where it's hard to convince your team of using it, but because its hard they is less contribution.
As for llvm, you might be right, I'm not an expert in that particular field. But it seems there is a lot of already backed in stuff you'll have for free. Like Wasm, debugging, optimization. We'll see how the Crystal project manage to get Wasm and how llvm will help them.
The reason it's easier in Rust is because someone that's passionate about WASM came along and did the work up front to document everything and make it as simple to compile to WASM as possible, the same is certainly possible in Nim, just need someone to put in some work :)