xen: introduce xen_has_percpu_evtchn()

xen_vector_callback_enabled is x86 specific and availability of
per-cpu event channel delivery differs on other architectures.

Introduce a new helper to check if there's support for per-cpu event
channel injection.

Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D29402