runtime/wasm: provide dummy runtime.setEventHandler

Event handlers aren't supported yet. This commit gets syscall/js to
compile for the WebAssembly target.

src/runtime/runtime_wasm.go

@@ -37,6 +37,11 @@ func putchar(c byte) {
 	resource_write(stdout, &c, 1)
 }
 
+//go:linkname setEventHandler syscall/js.setEventHandler
+func setEventHandler(fn func()) {
+	// TODO
+}
+
 //go:export go_scheduler
 func go_scheduler() {
 	scheduler()