IPL_NONE doesn't really exist. The corresponding bit in the

IPL_NONE doesn't really exist.  The corresponding bit in the
per-processor soft interrupt register is used for hardware clock-tick
counter interrupts.  So make smp_signotify() use IPL_SOFTINT instead
such that we don't unintentionally trigger a spurious clock interrupt.
This seems to fix the timekeeping anomalies on the t1k.

tested by art@