wasm: add //go:wasmexport support to js/wasm

This adds support for //go:wasmexport with `-target=wasm` (in the
browser). This follows the //go:wasmexport proposal, meaning that
blocking functions are not allowed.

Both `-buildmode=default` and `-buildmode=c-shared` are supported. The
latter allows calling exported functions after `go.run()` has returned.

testdata/wasmexport.js

@@ -0,0 +1,40 @@
+require('../targets/wasm_exec.js');
+
+function runTests() {
+    let testCall = (name, params, expected) => {
+        let result = go._inst.exports[name].apply(null, params);
+        if (result !== expected) {
+            console.error(`${name}(...${params}): expected result ${expected}, got ${result}`);
+        }
+    }
+
+    // These are the same tests as in TestWasmExport.
+    testCall('hello', [], undefined);
+    testCall('add', [3, 5], 8);
+    testCall('add', [7, 9], 16);
+    testCall('add', [6, 1], 7);
+    testCall('reentrantCall', [2, 3], 5);
+    testCall('reentrantCall', [1, 8], 9);
+}
+
+let go = new Go();
+go.importObject.tester = {
+    callOutside: (a, b) => {
+        return go._inst.exports.add(a, b);
+    },
+    callTestMain: () => {
+        runTests();
+    },
+};
+WebAssembly.instantiate(fs.readFileSync(process.argv[2]), go.importObject).then((result) => {
+    let buildMode = process.argv[3];
+    if (buildMode === 'default') {
+        go.run(result.instance);
+    } else if (buildMode === 'c-shared') {
+        go.run(result.instance);
+        runTests();
+    }
+}).catch((err) => {
+    console.error(err);
+    process.exit(1);
+});