Unsynchronized TSCs on the host require special handling in bhyve:

- use clock_gettime(2) as the time base for the emulated ACPI timer instead
  of directly using rdtsc().

- don't advertise the invariant TSC capability to the guest to discourage it
  from using the TSC as its time base.

Discussed with:	jhb@ (about making 'smp_tsc' a global)
Reported by:	Dan Mack on freebsd-virtualization@
Obtained from:	NetApp