wasm: call __stdio_exit on exit

This flushes stdio, so that functions like puts and printf write out all
buffered data even if the output isn't connected to a terminal.

For discussion, see:
https://github.com/bytecodealliance/wasmtime/issues/7833

src/runtime/runtime_wasm_js.go

@@ -18,6 +18,7 @@ func _start() {
 
 	wasmNested = true
 	run()
+	__stdio_exit()
 	wasmNested = false
 }