cmd/bent: add flag to disable build time reporting

We do not want these benchmarks in cmd/bench because we only run builds
once and thus do not get statistically significant results anyway.

For golang/go#57770.

Change-Id: I75987416458f826ee0a3e71c69f36fd48206eb77
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/463196
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
3 files changed