x86: (allow to) override LIST_POISON*

x86: (allow to) override LIST_POISON*

Having these point into space not controlled by the hypervisor provides
an unnecessary attack surface. Allow architectures to override them and
utilize that override to make them non-canonical addresses (thus
causing #GP rather than #PF when dereferenced).

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>