Feature Request: Control hardware concurrency reported to C/C++

Currently Emscripten generates the following syscall stub, which hardcodes `navigator.hardwareConcurrency` as CPU count reported back to C/C++.

```js
case 84:
{
if (typeof navigator === "object") return navigator["hardwareConcurrency"] || 1;
return 1;
}
```

It'd be great to have more control over this. For example in case of LLDB/LLVM, the thread pools will always use this concurrency, and thus they'll try to spawn 96 workers on my CloudTop, unless I manually hack LLVM :-)