benchtests: Add modff benchtest

It adds four ranges, which is how the generic implementation handles
normal numbers:

  1. Random inputs in the range [0.0, 1.0];
  2. Random inputs in the range [1.0, (float)(1U << 23)];
  3. Random inputs in the range [(float)(1U << 23), FLT_MAX];
  4. Random integral inputs in the range [0.0, (float)(1U << 23)].

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

benchtests/Makefile

@@ -104,6 +104,7 @@ bench-math := \
   lrint \
   lrintf \
   modf \
+  modff \
   nearbyint \
   nearbyintf \
   pow \