benchtests: Build ffs and ffsl benchtests with -fno-builtin

Without this flag it is possible that the compiler will optimize
away the calls to ffs/ffsll.

ChangeLog:

2014-04-01  Will Newton  <will.newton@linaro.org>

	* benchtests/Makefile (CFLAGS-bench-ffs.c): Add
	-fno-builtin.  (CFLAGS-bench-ffsll.c): Likewise.