libc: Flush all open streams on exit

This does nothing for now, but prepares for buffering.

libc/src/atexit.cpp

@@ -1,3 +1,4 @@
+#include <stdio.h>
 #include <stdlib.h>
 
 typedef void (*atexit_func_t)(void);
@@ -23,6 +24,8 @@ extern "C"
     {
         while (atexit_registered_funcs--) { atexit_funcs[atexit_registered_funcs](); }
 
+        fflush(NULL);
+
         _Exit(status);
     }
 }