benchtests: Add benchtests for ffs and ffsll

Add benchtests for ffs and ffsll. There is no benchtest for ffsl as
it is identical to one of the other functions.

2014-03-31  Will Newton  <will.newton@linaro.org>

	* benchtests/Makefile (bench): Add ffs and ffsll to list
	of tests.
	* benchtests/ffs-inputs: New file.
	* benchtests/ffsll-inputs: Likewise.