runtime: add BenchmarkScanStack
runtime: add BenchmarkScanStack

There are many possible stack scanning benchmarks,
but this one is at least a start.

cpuprofiling shows about 75% of CPU in func scanstack.

Change-Id: I906b0493966f2165c1920636c4e057d16d6447e0
Reviewed-on: https://go-review.googlesource.com/105535
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

cloudflare/go,golang/go,josharian/go,CAFxX/go,akutz/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,golang/go,golang/go,CAFxX/go,akutz/go,golang/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,cloudflare/go,akutz/go,josharian/go,josharian/go,josharian/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go