Use the correct way to check for Asyncify

Asyncify is a scoped object within emscripten runtime main function.
Use of EM_JS seems inevitable at this point.

Change-Id: I4852a7054a5e01583139cb54a0e14f604ac8ed13
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/corelib/platform/wasm/qstdweb.cpp

@@ -295,7 +295,9 @@ void WebPromiseManager::adoptPromise(emscripten::val target, PromiseCallbacks ca
         registerPromise(std::move(allocation), std::move(callbacks));
     });
 }
-}  // namespace
+
+EM_JS(bool, jsHaveAsyncify, (), { return typeof Asyncify !== "undefined"; });
+} // namespace
 
 ArrayBuffer::ArrayBuffer(uint32_t size)
 {
@@ -669,7 +671,7 @@ namespace Promise {
 
 bool haveAsyncify()
 {
-    static bool HaveAsyncify = !emscripten::val::global("Asyncify").isUndefined();
+    static bool HaveAsyncify = jsHaveAsyncify();
     return HaveAsyncify;
 }