[wasm] Rename _WASMEXP_ object to Wasm.

test/mjsunit/wasm/embenchen/box2d.js

 // Modified embenchen to direct to asm-wasm.
 // Flags: --expose-wasm
 
 var EXPECTED_OUTPUT =
   /frame averages: .+ \+- .+, range: .+ to .+ \n/;
 var Module = {
   arguments: [1],
   print: function(x) {Module.printBuffer += x + '\n';},
   preRun: [function() {Module.printBuffer = ''}],
   postRun: [function() {
@@ skipping 6020 matching lines @@
   }
 }
 
 function asmPrintInt(x, y) {
   Module.print('int ' + x + ',' + y);// + ' ' + new Error().stack);
 }
 function asmPrintFloat(x, y) {
   Module.print('float ' + x + ',' + y);// + ' ' + new Error().stack);
 }
 // EMSCRIPTEN_START_ASM
-var asm = _WASMEXP_.instantiateModuleFromAsm((function Module(global, env, buffer) {
+var asm = Wasm.instantiateModuleFromAsm((function Module(global, env, buffer) {
   'use asm';
   var HEAP8 = new global.Int8Array(buffer);
   var HEAP16 = new global.Int16Array(buffer);
   var HEAP32 = new global.Int32Array(buffer);
   var HEAPU8 = new global.Uint8Array(buffer);
   var HEAPU16 = new global.Uint16Array(buffer);
   var HEAPU32 = new global.Uint32Array(buffer);
   var HEAPF32 = new global.Float32Array(buffer);
   var HEAPF64 = new global.Float64Array(buffer);
 
@@ skipping 14264 matching lines @@
 }
 
 // shouldRunNow refers to calling main(), not run().
 var shouldRunNow = true;
 if (Module['noInitialRun']) {
   shouldRunNow = false;
 }
 
 
 run([].concat(Module["arguments"]));