ASSERTIONS mode should tell the user when trying to use pthreads w/o -pthread

I just tried building [simple C++ example](https://web.dev/webassembly-threads/#c-2) that uses threads and trying to run, getting an anonymous abort somewhere in Emscripten internals:

```js
> node temp
Before the thread
Aborted(native code called abort())
/home/rreverser/temp.js:147
throw ex;
^

RuntimeError: Aborted(native code called abort())
at abort (/home/rreverser/temp.js:990:11)
at _abort (/home/rreverser/temp.js:1304:7)
at std::__2::__throw_system_error(int, char const*) (:wasm-function[1689]:0x24cef)
at std::__2::thread::thread(main::$_0&&) (:wasm-function[15]:0x1167)
at __original_main (:wasm-function[14]:0xfff)
at main (:wasm-function[135]:0x3e75)
at /home/rreverser/temp.js:1034:22
at callMain (/home/rreverser/temp.js:4806:15)
at doRun (/home/rreverser/temp.js:4860:23)
at run (/home/rreverser/temp.js:4875:5)
```

After a few minutes of cleaning up `emcc` cache & rebuilding (in case it was my local pthread-related experiments that broke something), I finally realised that I just forgot to add `-pthread` flag when building 🤦‍♂️

So Emscripten linked it with the `library_pthread_stub.js` instead, which has most operations stubbed out.

I think in the default `ASSERTIONS` mode it would be worth to either print a warning or even throw an early error when `pthread_create` or other unsupported operations are invoked.

Also, generally, `__throw_system_error` probably shouldn't ignore the message parameter in non-exceptions mode and instead pass it to be printed by JS here: https://github.com/emscripten-core/emscripten/blob/2e02d0eba92c9c7489bed38eff9bd1ee61e93229/system/lib/libcxx/src/system_error.cpp#L294. The cost of passing it would be negligible, but error message at least wouldn't be lost.