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_wasip1.go

@@ -19,6 +19,7 @@ func _start() {
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
 	run()
+	__stdio_exit()
 }
 
 // Read the command line arguments from WASI.