x86/madt: allow Xen guest to use x2APIC mode

The old bogus Xen versions that would deliver a GPF when writing to
the LAPIC MSR are likely retired, so it's safe to enable x2APIC
unconditionally now if available.

Tested by: avg
Reviewed by: kib
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D33877