runtime: add countAlloc benchmark

runtime: add countAlloc benchmark

This change adds a small microbenchmark for (*mspan).countAlloc, which
we're about to replace. Admittedly this isn't a critical piece of code,
but the benchmark was useful in understanding the performance change.

Change-Id: Iea93c00f571ee95534a42f2ef2ab026b382242b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/224438
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>