Ensure that profile and tick probes provide a non-zero PC value.

The idle thread may process callouts while reloading the timer in
cpu_activeclock(). In this case, provide a representative value, &cpu_idle,
instead of 0 for args[0] so that the active thread can be more easily
identified from the probe.

This addresses intermittent failures of the profile-n/tst.argtest.d test.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D10651