Compute CPU speed(MHz) and loop multiplier for DELAY() based on
counting empty loop between mcclock ticks. New global: cpu_mhz.
Change pmax/pmax/machdep.c to build baseboard model names from cpu_mhz.
Set 'cpuspeed' for more realistic DELAY() on mips3 models.
Mips CPU constants, testing, and calibration from D. Sean Davidson
<davidson@zk3.dec.com> and Simon Burge <simonb@telstra.com.au>.