MFC r249625:
Introduce kern.timecounter.smp_tsc_adjust tunable (disabled by default) and
respective functionality, allowing to synchronize TSC on APs to match BSP's
during boot.  It may be unsafe in general case due to theoretical chance of
later drift if CPUs are using different clock rate or source, but it allows
to use TSC in some cases when difference caused by some initialization bug,
while TSCs are known to increment synchronously.