Add ExcludeArch tag to ensure that grub2-greenbot isn't built for s390x
    
    Greenboot depends on the GRUB bootloader but this isn't used for s390x,
    so it is not useful to have this package built on that architecture.
    
    Also, the noarch greenboot-grub2 package will have a broken dependency
    since it requires grub2-tools-minimal which is not built on s390x arch.
    
    Resolves: rhbz#1915241
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>