cirrus-ci: introduce some basic FreeBSD testing

cirrus-ci: introduce some basic FreeBSD testing

Cirrus-CI supports FreeBSD natively, so introduce some build tests on
FreeBSD.

The Cirrus-CI requires a Github repository in order to trigger the
tests.

A sample run output can be seen at:

https://github.com/royger/xen/runs/1962451343

Note the FreeBSD 11 task fails to build QEMU and is not part of this
patch.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>