runtime: improve StackCopy benchmarks
runtime: improve StackCopy benchmarks

Make the StackCopyNoCache test easier to read.

Add a StackCopyPtr test that actually has some pointers
that need adjusting.

Change-Id: I5b07c26f40cb485c9de97ed63fac89a9e6f36650
Reviewed-on: https://go-review.googlesource.com/104195
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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