x86: Prefer consistent naming for loader tunables

The following loader tunables do have corresponding sysctl MIBs but
with inconsistent naming. That may be historical reason. Let's prefer
consistent naming for them so that it will be easier to maintain.

1. hw.dmar.timeout -> hw.iommu.dmar.timeout
2. hw.lapic_eoi_suppression -> hw.apic.eoi_suppression
3. hw.lapic_tsc_deadline -> hw.apic.timer_tsc_deadline
4. hw.x2apic_enable -> hw.apic.x2apic_mode

Those tunables are for field debugging, no need to keep old names for
compatibility.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42248

(cherry picked from commit 12cce5994b92f8235f379d660ccb28da8e69f55b)