WASM: exec() fails, even when ASYNCIFY is enabled

Related: https://bugreports.qt.io/browse/QTBUG-111329

Tried my best to follow the guidelines for getting a WASM build up and running with asyncify support enabled to allow e.g. dialogs to work (https://doc.qt.io/qt-6/wasm.html#asyncify).

However, using the 6.5.0 release build (both with and without multithreaded support), a simple test application freezes once the `exec()` call occurs. As i read it: 

> `As of Qt 6.4, Asyncify support is enabled in the binary package`

implies that Qt 6.5 should have Asyncify support built-in, in the distributed package.

If i do not enable `-sASYNCIFY` in the CMakeLists file, the dialog will show, but then things start to go awry when the dialog (attempts to) return, as per all of the other forum posts and bugs on this topic.

To reproduce: https://gist.github.com/mortbopet/a1641dfc4c0f09d8002232058ea8a7e8 built using Qt 6.5.0 WASM.