benchtests: Reallocate buffers for memset

Keeping the same buffers along with copying the same size of data into
the same location means that the first routine is typically the
slowest since it has to bear the cost of fetching data into to cache.
Reallocating buffers stabilizes numbers by a bit.

	* benchtests/bench-string.h (realloc_bufs): New function.
	(test_init): Call it.
	* benchtests/bench-memset-large.c (do_test): Likewise.
	* benchtests/bench-memset.c (do_test): Likewise.