Add a new randomized memset test similar to bench-random-memcpy. Instead of
repeating the same call to memset over and over again, it times a large number
of different inputs. The distribution of memset length and alignment is based
on SPEC2017 (length up to 4096 and alignment up to 64).
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
