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

sys/x86/x86/local_apic.c

@@ -855,7 +855,8 @@ native_lapic_intrcnt(void *dummy __unused)
 
 	STAILQ_FOREACH(pc, &cpuhead, pc_allcpu) {
 		la = &lapics[pc->pc_apic_id];
-		KASSERT(la->la_present, ("missing APIC structure"));
+		if (!la->la_present)
+		    continue;
 
 		snprintf(buf, sizeof(buf), "cpu%d:timer", pc->pc_cpuid);
 		intrcnt_add(buf, &la->la_timer_count);