TIMER: Run simulator pre-calibration for a minimum of 100ms

Faster host systems today can get very fast instruction execution rates
for a short duration calibration test.  These may be skewed by round
off error, so we now run the calibration for a minimum of 100ms.

sim_timer.c

@@ -3459,8 +3459,10 @@ sim_switches = saved_switches;
 sim_cancel (&SIM_INTERNAL_UNIT);
 sim_activate (&precalib_unit, sim_precalibrate_ips);
 start = sim_os_msec();
-sim_instr();
-end = sim_os_msec();
+do {
+    sim_instr();
+    end = sim_os_msec();
+    } while ((end - start) < SIM_PRE_CALIBRATE_MIN_MS);
 sim_precalibrate_ips = (int32)(1000.0 * (sim_precalibrate_ips / (double)(end - start)));
 
 for (tmr=0; tmr<=SIM_NTIMERS; tmr++) {

sim_timer.h

@@ -78,6 +78,7 @@ int clock_gettime(int clock_id, struct timespec *tp);
 
 #define SIM_INITIAL_IPS 5000000                     /* uncalibrated assumption */
                                                     /* about instructions per second */
+#define SIM_PRE_CALIBRATE_MIN_MS    100             /* minimum time to run precalibration activities */
 
 #define SIM_IDLE_CAL    10                          /* ms to calibrate */
 #define SIM_IDLE_STMIN  2                           /* min sec for stability */