wasm: don't call dlopen() on static builds

Make QLibraryPrivate::load_sys() return false on static
wasm builds.

Emscripten does not support dlopen() in this configuration;
calling it will abort the program. By returning false
we give QLibrary users an opportunity to handle the
error.

Task-number: QTBUG-109076
Pick-to: 6.5
Change-Id: I32d1fde04cc54d1622e0743712b6372b023aa006
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Mikołaj Boc <Mikolaj.Boc@qt.io>

src/corelib/plugin/qlibrary_unix.cpp

@@ -90,6 +90,11 @@ QStringList QLibraryPrivate::prefixes_sys()
 
 bool QLibraryPrivate::load_sys()
 {
+#if defined(Q_OS_WASM) && defined(QT_STATIC)
+    // emscripten does not support dlopen when using static linking
+    return false;
+#endif
+
     QMutexLocker locker(&mutex);
     QString attempt;
     QFileSystemEntry fsEntry(fileName);