Simplify use of new Function in embind. NFC (emscripten-core#23954)

Passing the function body explicitly seems less magical, and also saves
a little on code size.

Author: sbc100

src/lib/libembind.js

@@ -858,9 +858,7 @@ var LibraryEmbind = {
 #else
 
     let [args, invokerFnBody] = createJsInvoker(argTypes, isClassMethodFunc, returns, isAsync);
-    args.push(invokerFnBody);
-    var invokerFn = new Function(...args)(...closureArgs);
-
+    var invokerFn = new Function(...args, invokerFnBody)(...closureArgs);
 #endif
 #endif
     return createNamedFunction(humanName, invokerFn);

src/lib/libemval.js

@@ -381,8 +381,7 @@ var LibraryEmVal = {
     functionBody +=
       "};\n";
 
-    params.push(functionBody);
-    var invokerFunction = new Function(...params)(...args);
+    var invokerFunction = new Function(...params, functionBody)(...args);
 #endif
     var functionName = `methodCaller<(${types.map(t => t.name).join(', ')}) => ${retType.name}>`;
     return emval_addMethodCaller(createNamedFunction(functionName, invokerFunction));

test/code_size/embind_hello_wasm.json

@@ -1,10 +1,10 @@
 {
   "a.html": 552,
   "a.html.gz": 380,
-  "a.js": 9482,
-  "a.js.gz": 4164,
+  "a.js": 9473,
+  "a.js.gz": 4157,
   "a.wasm": 7348,
   "a.wasm.gz": 3378,
-  "total": 17382,
-  "total_gz": 7922
+  "total": 17373,
+  "total_gz": 7915
 }