[SPLIT_MODULE] Fix imports into deferred module when loading (#25621)

Fix an error when attempted to reserve a new memory space in the
deferred module when building with MEMORY64 + SPLIT_MODULE.
```
TypeError: Cannot convert <func ptr> to a BigInt
```

In a mem64 build, we call applySignatureConversions on `malloc()` in the
wasmExports. `applySignatureConversions()` wraps the exports and
converts the return value to a javascript number. The wasmExports is
passed into the deferred module during instantiation so when we call a
malloc in the deferred module, we'd be calling the wrapped function,
hitting into this error.

Furthermore, we are making a direct call when calling a malloc, so
strictly speaking, we should not need to go through the JS trampoline.

Author: Mintyboi

src/lib/libasync.js

@@ -523,7 +523,7 @@ addToLibrary({
   _load_secondary_module: async function() {
     // Mark the module as loading for the wasm module (so it doesn't try to load it again).
     wasmExports['load_secondary_module_status'].value = 1;
-    var imports = {'primary': wasmExports};
+    var imports = {'primary': wasmRawExports};
     // Replace '.wasm' suffix with '.deferred.wasm'.
     var deferred = wasmBinaryFile.slice(0, -5) + '.deferred.wasm';
     await instantiateAsync(null, deferred, imports);

src/postamble.js

@@ -289,6 +289,9 @@ function checkUnflushedContent() {
 #endif // ASSERTIONS
 
 var wasmExports;
+#if SPLIT_MODULE
+var wasmRawExports;
+#endif
 
 #if MODULARIZE == 'instance'
 // In MODULARIZE=instance mode we delay most of the initialization work until

src/preamble.js

@@ -514,7 +514,7 @@ var splitModuleProxyHandler = {
 #if RUNTIME_DEBUG
             dbg(`placeholder function called: ${base}`);
 #endif
-            var imports = {'primary': wasmExports};
+            var imports = {'primary': wasmRawExports};
             // Replace '.wasm' suffix with '.deferred.wasm'.
             loadSplitModule(secondaryFile, imports, base);
 #if RUNTIME_DEBUG
@@ -734,6 +734,9 @@ function getWasmImports() {
     reportUndefinedSymbols();
 #endif
 
+#if SPLIT_MODULE
+  wasmRawExports = wasmExports;
+#endif
 #if MEMORY64 || CAN_ADDRESS_2GB
     wasmExports = applySignatureConversions(wasmExports);
 #endif

test/other/test_split_module.c

@@ -7,4 +7,12 @@ int foo() {
 
 EMSCRIPTEN_KEEPALIVE void say_hello() {
   printf("Hello! answer: %d\n", foo());
+
+  void* ptr = malloc(10);
+  if (ptr == NULL) {
+    return;
+  }
+  
+  free(ptr);
+  ptr = NULL;
 }