benchtests: Add fmaxf/fminf benchmarks

This patch adds fmaxf and fminf benchtests.  It is based on
math/s_fmax_template.c implementation which checks for basically four
different classes:

  1. if x is greater or equal than y.
  2. if x is less than y.
  3. if x or y is signaling.
  4. if y is nan.

Cases 1 and 2 are used for default input number (by mixing normal double
numbers and infinity), while case 3 and 4 are used each for on for a
benchmark class.

Checked on x86_64-linux-gnu and powerpc64-linux-gnu.

	* benchtests/Makefile (bench-math): Add fminf and fmaxf.
	(CFLAGS-bench-fmaxf.c): New rule.
	(CFLAGS-bench-fminf.c): Likewise.
        * benchtests/fmaxf-inputs: New file.
        * benchtests/fminf-inputs: Likewise.