benchtests: Make memset benchmarks print json

benchtests: Make memset benchmarks print json

Make the memset benchmarks (bench-memset and bench-memset-large) print
their output in JSON so that they can be evaluated using the
compare_strings.py script.

	* benchtests/bench-memset-large.c: Print output in JSON
	format.
	* benchtests/bench-memset.c: Likewise.