lapic: skip setting intrcnt if lapic is not present

Instead of panicking. Legacy PVH mode doesn't provide a lapic, and
since native_lapic_intrcnt is called unconditionally this would cause
the assert to trigger. Change the assert into a continue in order to
take into account the possibility of systems without a lapic.

Reviewed by: jhb
Approved by: re (gjb)
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D17015