sort: use a very fast random generator for benchmarks
sort: use a very fast random generator for benchmarks

Adapted from https://codereview.appspot.com/11564044.

Fixes breakage of darwin-amd64-race builder.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13352045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang