Initialize DTrace hrtimer frequency during SI_SUB_CPU on i386 and amd64.

This allows the hrtimer to be used earlier during boot. This is required
for boot-time DTrace: anonymous enablings are created during
SI_SUB_DTRACE_ANON, which runs before APs are started. In particular,
the DTrace deadman timer requires that the hrtimer be functional.

MFC after: 2 weeks