mem: Change prefetcher to use random_mt
mem: Change prefetcher to use random_mt

Prefechers has used rand() to generate random numers previously.
(transplanted from 63edd4a1243f3c8a4e652f308a735659fcc35520)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5