benchtests: Print json in memmove benchmark

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

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