Ensure compatibility with Safari for multi-environment ES6 builds

Use a different parameter name to prevent the variable from being
incorrectly hoisted in JavaScriptCore-based engines (such as Safari).
See: https://bugs.webkit.org/show_bug.cgi?id=223533.

Regressed for multi-environment ES6 builds since commit ce4c405
(Emscripten 3.1.27). `-sENVIRONMENT=web` is not affected, as that
would avoid the emit of this async function altogether.

Resolves: #18357.

Author: kleisauke

emcc.py

@@ -3767,9 +3767,10 @@ def modularize():
   if not settings.EXPORT_READY_PROMISE:
     return_value = '{}'
 
+  # FIXME: https://bugs.webkit.org/show_bug.cgi?id=223533
   src = '''
-%(maybe_async)sfunction(%(EXPORT_NAME)s) {
-  %(EXPORT_NAME)s = %(EXPORT_NAME)s || {};
+%(maybe_async)sfunction(_%(EXPORT_NAME)s) {
+  %(EXPORT_NAME)s = _%(EXPORT_NAME)s || {};
 
 %(src)s