`uname` should return something more meaningful

`uname(2)` is currently delegated to [`library_syscall.js`](https://github.com/emscripten-core/emscripten/blob/ae8649296055ebe66195d06a8040e93db10a763a/src/library_syscall.js#L786).

At present, it hard codes the following values:
```
copyString('sysname', 'Emscripten');
copyString('nodename', 'emscripten');
copyString('release', '1.0');
copyString('version', '#1');
copyString('machine', 'x86-JS');
```

It might be nice to return something more meaningful here.

Suggestions:
* If NodeJS, provide `product.versions.node`
* If browser, provide `navigator.userAgent`
* Encode the Emscripten toolchain version here, as, in some sense, the version of the toolchain (e.g. 2.0.12) is also representative of some amount of the glue code / runtime