PDP10: Recompute tmxr delay factor regularly

This will not change much, but it may be useful for sane idling depending
on the way the running OS actually uses the interval timer for clock ticks.

Some discussion about this in #713

PDP10/pdp10_tim.c

@@ -391,7 +391,11 @@ else {
     sim_activate_after (uptr, tick_in_usecs);           /* reactivate unit */
     tmr_poll = sim_activate_time (uptr) - 1;
     }
-tmxr_poll = (int32)(sim_timer_inst_per_sec () / clk_tps);/* set mux poll */
+
+/* tmxr is polled every tim_mult clks.  Compute the divisor matching the target. */
+tim_mult = (clk_tps <= TIM_TMXR_FREQ) ? 1 : (clk_tps / TIM_TMXR_FREQ) ;
+   
+tmxr_poll = tim_mult * (int32)(sim_timer_inst_per_sec () / clk_tps);/* set mux poll */
 tim_incr_base (tim_base, tim_period);                   /* incr time base based on period of expired interval */
 tim_period = tim_new_period;                            /* If interval has changed, update period */
 apr_flg = apr_flg | APRF_TIM;                           /* request interrupt */