Add support for the go runtime on openbsd/amd64. This is based on
the existing freebsd runtime.

Threads are implemented using OpenBSD's rthreads, which are currently
disabled by default, however can be enabled via the kern.rthreads
sysctl.

For now, cgo is disabled.

R=rsc
CC=golang-dev
https://golang.org/cl/4815067