Fix getting tunable values on big-endian (BZ #21109)

The code to set value passed a tunable_val_t, which when cast to
int32_t on big-endian gives the wrong value.  Instead, use
tunable_val_t.numval instead, which can then be safely cast into
int32_t.

elf/dl-tunables.c

@@ -455,6 +455,8 @@ __tunable_set_val (tunable_id_t id, void *valp, tunable_callback_t callback)
   if (cur->strval == NULL)
     return;
 
+  /* Caller does not need the value, just call the callback with our tunable
+     value.  */
   if (valp == NULL)
     goto cb;