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 #5837.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/11564044

src/pkg/regexp/exec_test.go

@@ -9,7 +9,6 @@ import (
 	"compress/bzip2"
 	"fmt"
 	"io"
-	"math/rand"
 	"os"
 	"path/filepath"
 	"regexp/syntax"
@@ -643,11 +642,17 @@ func makeText(n int) []byte {
 		return text[:n]
 	}
 	text = make([]byte, n)
+	x := ^uint32(0)
 	for i := range text {
-		if rand.Intn(30) == 0 {
+		x += x
+		x ^= 1
+		if int32(x) < 0 {
+			x ^= 0x88888eef
+		}
+		if x%31 == 0 {
 			text[i] = '\n'
 		} else {
-			text[i] = byte(rand.Intn(0x7E+1-0x20) + 0x20)
+			text[i] = byte(x%(0x7E+1-0x20) + 0x20)
 		}
 	}
 	return text