cpu: native: adapt xtimer backoff values to periph/timer change

cpu/native/include/periph_conf.h

@@ -54,6 +54,14 @@
  */
 #define XTIMER_OVERHEAD 14
 #define XTIMER_USLEEP_UNTIL_OVERHEAD 1
+
+/* timer_set_absolute() has a high margin for possible underflow if set with
+ * value not far in the future. To prevent this, we set high backoff values
+ * here.
+ */
+#define XTIMER_BACKOFF      200
+#define XTIMER_ISR_BACKOFF  200
+
 /** @} */
 
 #ifdef __cplusplus