Use -fno-builtin for sqrt benchmark.

This patch makes the sqrt benchmark use -fno-builtin, as already done
for benchmarks of ffs and ffsll, so that it actually benchmarks the
glibc function as (presumably) intended even in the presence of the
compiler inlining sqrt.

Tested for x86_64 and also used for benchmarking my ARM sqrt patch.

	* benchtests/Makefile (CFLAGS-bench-sqrt.c): New variable.