benchmarks/fpu: improve timing precision to milliseconds

Change FPU benchmark timing from seconds to milliseconds for better accuracy.
This allows for more precise measurement of test cycles, especially for
shorter test runs that previously completed within a single second.

Signed-off-by: makejian <makejian@xiaomi.com>

Author: ThePassionate

benchmarks/whetstone/whetstone.c

@@ -143,10 +143,11 @@ int main(int argc, FAR char *argv[])
   /* added for this version */
 
   long loopstart;
-  long startsec;
-  long finisec;
+  long startmsec;
+  long finimsec;
   float KIPS;
   int continuous;
+  struct timespec ts;
 
   loopstart = 1000;    /* see the note about loop below */
   continuous = 0;
@@ -175,7 +176,8 @@ int main(int argc, FAR char *argv[])
 
   /* Start benchmark timing at this point. */
 
-  startsec = time(0);
+  clock_gettime(CLOCK_REALTIME, &ts);
+  startmsec = ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
 
   /* The actual benchmark starts here. */
 
@@ -431,7 +433,8 @@ int main(int argc, FAR char *argv[])
 
   /* Stop benchmark timing at this point. */
 
-  finisec = time(0);
+  clock_gettime(CLOCK_REALTIME, &ts);
+  finimsec = ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
 
   /* Performance in Whetstone KIP's per second is given by
    *
@@ -441,16 +444,16 @@ int main(int argc, FAR char *argv[])
    */
 
   printf("\n");
-  if (finisec - startsec <= 0)
+  if (finimsec - startmsec <= 0)
     {
       printf("Insufficient duration- Increase the loop count\n");
       return 1;
     }
 
-  printf("Loops: %ld, Iterations: %d, Duration: %ld sec.\n",
-                                loop, ii, finisec - startsec);
+  printf("Loops: %ld, Iterations: %d, Duration: %ld millisecond.\n",
+                                loop, ii, finimsec - startmsec);
 
-  KIPS = (100.0 * loop * ii) / (float)(finisec - startsec);
+  KIPS = (100.0 * loop * ii) / ((float)(finimsec - startmsec) * 1000);
   if (KIPS >= 1000.0)
     {
       printf("C Converted Double Precision Whetstones: %.1f MIPS\n",