Add support for OpenBSD

Add support for OpenBSD

- Add OpenBSD to syscall compatibility header as appropriate.
- Add function for retrieving the thread id in urcu_get_thread_id().
- Rely on pthread cond variables for futex compatibility.

It builds on all of our archs and fully run time tested on amd64.

Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I5cca5962ba3dc3113c9bd12e544b6e6f77dfdb61