Use per-cpu values for base and last in tc_cpu_ticks(). The values

Use per-cpu values for base and last in tc_cpu_ticks().  The values
are updated lockess, different CPUs write its own view of timecounter
state.  The critical section is done for safety, callers of
tc_cpu_ticks() are supposed to already enter critical section, or to
own a spinlock.

The change fixes sporadical reports of too high values reported for
the (W)CPU on platforms that do not provide cpu ticker and use
tc_cpu_ticks(), in particular, arm*.

Diagnosed and reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week