Properly add OpenBSD support.

This patch allows 64-bit x86 OpenBSD machines to be properly detected, to allow rock to build.
OpenBSD's ar does not have the -T option, so disable it like Mac OS X.

The sdk/threading/native/ThreadUnix.ooc patch is obviously not correct, but
version (!apple && !openbsd)
and
version (apple || openbsd)
did not work either. Guidance for that would be appreciated.

With this patch, rock builds and runs on OpenBSD/amd64.