Only calibrate ICR read loop when not in x2APIC mode. Run-time
switching between LAPIC modes is not supported, and there is no need
to wait for IPI ack in x2APIC mode. So the calibrated delay is only
needed for !x2APIC.

This saves around a second of boot time on the real hardware for
x2APIC.

Sponsored by: The FreeBSD Foundation