Fix SPY so that it samples even if not blocking

[rmkaplan]

Running WITH-SPY/SPY-TREE reports that it has collected no samples if run on a function that doesn't periodically BLOCK.

[nbriggs]

I'm surprised that this is so. Maiko generates calls in the "periodic interrupt" context, and I would have expected those to run independent of whether there is a call to BLOCK.

        /*** If SPY is running, check to see if it ***/
        /*** needs an interrupt; give it one, if so. ***/
        if (*PERIODIC_INTERRUPT68k != NIL) {
          if (period_cnt > 0)
            period_cnt--;
          else {
            cause_interruptcall(PERIODIC_INTERRUPTFRAME_index);
            if (*PERIODIC_INTERRUPT_FREQUENCY68k == NIL)
              period_cnt = 0;
            else
              period_cnt =
                ((*PERIODIC_INTERRUPT_FREQUENCY68k & 0xffff) * 1000000ULL) / (60 * TIMER_INTERVAL);
            /* number of 1/60 second periods between interrupts (really??)
            TIMER_INTERVAL is the number of microseconds between timer interrupts.
            */
          }
        }

but there's something odd going on -- I ran a small test where I know maiko called the periodic interrupt function 1539 times, but SPY reported that it collected 51 samples. Is SPY doing some of its own filtering before recording a sample?