benchtests: Add fmodf benchmark

1. Subnormals: 128 inputs.
2. Normal numbers with large exponent difference (|x/y| > 2^8):
   1024 inputs between FLT_MIN and FLT_MAX;
3. Close exponents (ey >= -103 and |x/y| < 2^8): 1024 inputs with
   exponents between -10 and 10.
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>