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>

.cirrus.yml

@@ -0,0 +1,26 @@
+# https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
+freebsd_template: &FREEBSD_TEMPLATE
+  environment:
+    APPEND_LIB: /usr/local/lib
+    APPEND_INCLUDES: /usr/local/include
+
+  install_script: pkg install -y seabios markdown gettext-tools gmake
+                                 pkgconf python libiconv bison perl5
+                                 yajl lzo2 pixman argp-standalone
+                                 libxml2 glib git
+
+  build_script:
+    - ./configure --with-system-seabios=/usr/local/share/seabios/bios.bin
+    - gmake -j`sysctl -n hw.ncpu` clang=y
+
+task:
+  name: 'FreeBSD 12'
+  freebsd_instance:
+    image_family: freebsd-12-2
+  << : *FREEBSD_TEMPLATE
+
+task:
+  name: 'FreeBSD 13'
+  freebsd_instance:
+    image_family: freebsd-13-0
+  << : *FREEBSD_TEMPLATE