[libc][armlibc] Fix getchar error after exit vi

- _sys_istty() add stdio as tty
 - without this, after exit vi program getchar will return -1, I don't know why it works

components/libc/compilers/armlibc/stubs.c

@@ -270,6 +270,9 @@ long _sys_flen(FILEHANDLE fh)
 
 int _sys_istty(FILEHANDLE fh)
 {
+    if((STDIN <= fh) && (fh <= STDERR))
+        return 1;
+    else
         return 0;
 }