benchtests: Reallocate buffers for every strncmp implementation

Don't reuse buffers for different strncmp implementations since the
earlier implementation will end up warming the cache for the later
one.  Eventually there should be a more elegant way to do this.

	* benchtests/bench-strncmp.c (do_test_limit): Reallocate buffers
	for every implementation.
	(do_test): Likewise.