benchtests: Add fmod benchmark

Add three different dataset, from random floating point numbers:

1. Subnormals: 128 inputs.
2. Normal numbers with large exponent difference (|x/y| > 2^52):
   1024 inputs between DBL_MIN and DBL_MAX;
3. Close exponents (ey >= -907 and |x/y| < 2^52): 1024 inputs with
   exponents between -10 and 10.
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

benchtests/Makefile

@@ -48,6 +48,7 @@ bench-math := \
   fmaxf \
   fmin \
   fminf \
+  fmod \
   hypot \
   hypotf \
   ilogb \