runtime: add a benchmark of Callers

runtime: add a benchmark of Callers

We're about to make major changes to tracebacks. We have benchmarks of
stack copying, but not of PC buffer filling, so add some that we can
track through these changes.

For #54466.

Change-Id: I3ed61d75144ba03b61517cd9834eeb71c99d74df
Reviewed-on: https://go-review.googlesource.com/c/go/+/472956
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>