How to compile wasm with with extern functions
Hello,

as far as i understand wasm, not only the native methods could be imported into js (that worked for me), but also javascript functions could be imported into wasm.

```js
const importObject = {
  imports: {
    myFunc: function() {
      console.log("hi");
    },
  },
};
const instance = new WebAssembly.Instance(mod, importObject);
```

For this, the functions are simply declared with extern in `C` and compiled, as far as I've seen.
**Is there a way to easily declare and compile this in nlvm?**

I tried the following:
```nim
proc myFunc() {.importc.}
```

result is
```bash
nlvm c --cpu:wasm32 --os:standalone --gc:none  --passl:--no-entry test.nim

wasm-ld: error: .../test.o: undefined symbol: myFunc
Error: linking failed
```

PS: this is a very great project. I've tried debugging with gdb and it works really well. Good work!
