benchtests: Bump start size since smaller sizes are noisy

Numbers for very small sizes (< 128B) are much noisier for non-cached
benchmarks like the walk benchmarks, so don't include them.

	* benchtests/bench-memcpy-walk.c (START_SIZE): Set to 128.
	* benchtests/bench-memmove-walk.c (START_SIZE): Likewise.
	* benchtests/bench-memset-walk.c (START_SIZE): Likewise.