wasm: correctly return from run() in wasm_exec.js

Instead of hanging forever, it should return the exit code from os.Exit.

src/runtime/runtime_tinygowasmp2.go

@@ -60,6 +60,13 @@ func syscall_Exit(code int) {
 	exit.Exit(code != 0)
 }
 
+func mainReturnExit() {
+	// WASIp2 does not use _start, instead it uses _initialize and a custom
+	// WASIp2-specific main function. So this should never be called in
+	// practice.
+	runtimePanic("unreachable: _start was called")
+}
+
 // TinyGo does not yet support any form of parallelism on WebAssembly, so these
 // can be left empty.