xen/tmem: do not allow XEN_TMEM on ARM64

tmem is not supported on arm or arm64 yet. Will revert this
once the Xen hypervisor supports it.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>