cirrus.yml: Add macOS continuous integration task

cirrus-ci.com also has the possibility to run CI tasks on macOS.
Since most of the QEMU developers do not have access to macOS yet,
let's add a CI pipeline for this operating system here, too.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Ed Maste <emaste@freebsd.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>