before the change, only support wasm app exit like:
```c
void *thread_routine(void *arg)
{
printf("Enter thread\n");
return NULL;
}
```
if call pthread_exit, it will crash:
```c
void *thread_routine(void *arg)
{
printf("Enter thread\n");
pthread_exit(NULL);
return NULL;
}
```
This commit lets both upstairs work correctly, test pass on stm32f103 mcu.
