Stop using re-assignment thunks when WASM_ASYNC_COMPILATION is enabled.

[sbc100]

In release mode, when WASM_ASYNC_COMPILATION is used (i.e the default) we generate little thunks for all wasm exports. They looks like this:

var _foo = Module['_foo'] = () => (_foo = Module['_cfoo'] = wasmExports['foo'])();

The idea here is that these would then get re-assigned on first use. However, there are several problem with this:

1. If somebody takes a reference early they will get a reference to wrapper that will not get re-assigned and will cause perpetual re-assignment:

// This runs early on
var myfunc = Module['foo'];

// Now my func will forever point to the wrapper:

myfunc();  // always goes via wrapper and always re-assigns.. strange
Module['foo'](); // now goes direct

2. The identity of these function changes which can be observable (since we don't this re-assignment thing in debug builds):

var myfunc = Module['foo'];
// later after instantiation and usage
if (myfunc == Module['foo']) // would always be true in debug builds, but can be false in release builds

See #23337

[sbc100]

(1) actually seems fairly bad since it perpetually mutates the Module and the globalThis.

[sbc100]

One possible solution would be to simply delay the assignments and just generate initially-undefined variables:

var _foo;  // Will simply be undefined until the instantiation is done.  As will `Module['_foo']`

In debug builds we could generate wrappers that simply fail if they are ever called:

var _foo = Module['_foo'] = reportModuleNotReady('_foo');

The major breakage here would be for folks that are currently taking references to _foo or Module['_foo'] before the module is ready.