Use the new SDM-approved way to serialize x2APIC MSR writes.

SDM editions 64 and below stated that it is enough to use MFENCe or
LFENCE to serialize x2APIC register writes. New edition 65 requires
either full serialization instruction or MFENCE;LFENCE sequence. Use
the later, FreeBSD needs serialization to ensure that writes done
before IPI request are visible to the target IPI CPU.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week