regexp: use a very fast random generator for benchmarks.
regexp: use a very fast random generator for benchmarks.

Calls into math/rand are very slow, especially under race
detector because of heap accesses.

go test -bench . -run none -benchtime .1s
Before: 23.0s
After:  17.4s

Fixes issue 5837.

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/11564044

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go