Since there is no so rich api in freertos like embedded system, simply set
CONFIG_HAS_CAP_ENTER to 1 to support posix file api for freertos.
Test file api in wasm app pass.
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.
