speed: show gocryptfs version

Output now looks like this

  $ gocryptfs -speed
  gocryptfs v1.7.1-38-gbe3b9df-dirty; go-fuse v2.0.2-57-gd1cfa17; 2020-04-13 go1.13.6 linux/amd64
  AES-GCM-256-OpenSSL 	 607.90 MB/s
  AES-GCM-256-Go      	 920.75 MB/s	(selected in auto mode)
  AES-SIV-512-Go      	 169.85 MB/s
  XChaCha20-Poly1305-Go	 794.30 MB/s

and has go version and arch information, which is important
when comparing results.

main.go

@@ -170,6 +170,7 @@ func main() {
 	}
 	// "-speed"
 	if args.speed {
+		printVersion()
 		speed.Run()
 		os.Exit(0)
 	}