LibC: Flush all file streams on exit

The POSIX standard specifies the following:

> If the main() function returns to its original caller, or if the
> exit() function is called, all open files are closed (hence all output
> streams are flushed) before program termination.

This means that flushing `stdin` and `stdout` only is not enough, as the
program might have pending writes in other file buffers too.

Now that we support `fflush(nullptr)`, we call that in `exit()` to flush
all streams. This fixes one of bash's generated headers not being
written to disk.

Userland/Libraries/LibC/stdlib.cpp

@@ -189,8 +189,7 @@ void exit(int status)
 
     extern void _fini();
     _fini();
-    fflush(stdout);
+    fflush(nullptr);
-    fflush(stderr);
 
 #ifndef _DYNAMIC_LOADER
     __pthread_key_destroy_for_current_thread();