cmd/bent: Change -R flag to supply ldflags for random linking, if unset.

It's quicker/better/easier to do it this way.

Change-Id: Ide76215c9ab4da02862ac171e0c4891e84a3e1e4
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/614776
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>