wasm: add macro for for emscritpen::async()

emscritpen:async() needs to be passed to embind exported
functions, if that function might suspend, and if JSPI
is enabled (and only if JSPI is enabled):

  emscripten::function("fn", &fn, emscritpen:async());

Add QT_WASM_EMSCRIPTEN_ASYNC macro which evaluates
to ",emscritpen::async()" (note comma) if JSPI is enabled,
and to nothing if not.

   emscripten::function("fn", &fn QT_WASM_EMSCRIPTEN_ASYNC);

This way we can configure Qt for either JSPI and non-jspi
builds. Incidentally the emscritpen::async() requirement
makes JSPI a Qt configure time feature, instead of an
app link time feature like for the current ASYNCIFY (1)
support.

Task-number: QTBUG-129749
Change-Id: I5bf820242c19a26018f4a3e06ee570e2d36ed993
Reviewed-by: Lorn Potter <lorn.potter@qt.io>
Reviewed-by: Piotr Wierciński <piotr.wiercinski@qt.io>

src/corelib/platform/wasm/qstdweb_p.h

@@ -34,6 +34,12 @@
 #include <emscripten/threading.h>
 #endif  // #if QT_CONFIG(thread)
 
+#if QT_CONFIG(wasm_jspi)
+# define QT_WASM_EMSCRIPTEN_ASYNC ,emscripten::async()
+#else
+# define QT_WASM_EMSCRIPTEN_ASYNC
+#endif
+
 QT_BEGIN_NAMESPACE
 
 class QMimeData;