The function xen_intr_alloc_and_bind_ipi() is only used on x86. As
sys/xen/xen_intr.h is common, it shouldn't include architecture-specific
pieces. Create sys/x86/include/xen/xen_intr_x86.h for the x86-only
function (previously there were several other x86-only functions here).
Commit 27c36a12f1584b53d2454dac238eeed3dedc82ba is an x86-only feature.
As such the variable should only exist on x86.