runtime: remove struct BitTarget

R=golang-dev
CC=dvyukov, golang-dev, rsc
https://codereview.appspot.com/7845043

Committer: Russ Cox <rsc@golang.org>
build: skip benchmarks on OpenBSD

They are making the build die. I want to be able to see that everything else is okay.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7546049