xen/intr: arm64: implement xen_arch_intr_intrcnt_init()

[ehem_freebsd_m5p.com]

Summary

Unfortunately this really looks like THE interrupt needs a full set of
IPI interrupt counters. For the moment, simply use a static throw-away
variable.

[ehem_freebsd_m5p.com]

I'm really unsure what this should look like. The interrupt really should have per-core counters since it is expected to occur on all cores which process Xen events. Yet the intrng code only has a single per-interrupt counter, thus appearing to implicitly assume any given interrupt will only occur on one core which the interrupt has been assigned to.

[ehem_freebsd_m5p.com]

I expect to abandon as a better solution has shown up, I'm simply waiting for confirmation before doing so.

[ehem_freebsd_m5p.com]

Abandoning as a better approach was identified and has now passed the first testing round.