benchtests: Add isnan/isinf/isfinite benchmark

	* benchtests/Makefile (bench-math): Add isnan, isinf, and isfinite.
	(CFLAGS-bench-isnan.c, CFLAGS-bench-isinf.c,
	CFLAGS-bench-isfinite.c): New rule.
	* benchtests/isnan-input: New file.
	* benchtests/isinf-input: New file.
	* benchtests/isfinite-input: New file.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>