Prototype support emscripten ASYNCIFY=2 (JSPI)

To make an app work with JSPI on Chrome:
1) Update to emscripten 3.1.43 (or above). Please mind that .43, as of Jun 26, is still unreleased. To get early access, apply all of the patches from sub-tasks to your emscripten.
2) Run chrome, go to chrome://flags, look up JSPI, enable the feature, restart the browser
3) Compile Qt for WebAssembly with -feature-wasm-exceptions
4) Modify your app to use the following linker switches:

-sDYNCALLS=1
-sASYNCIFY=2
-sASYNCIFY_EXPORTS=dynCall_*

5) Run the app in Chrome modified as in 1)
6) Enjoy JSPI!