i386: identifycpu(): only calibrate_cyclecounter() on primary CPU

i386: identifycpu(): only calibrate_cyclecounter() on primary CPU

On i386 during identifycpu(), we call calibrate_cyclecounter() for every
CPU in the system.  This is pointless: every new call clobbers the cpuspeed
measured during the prior call.  It is also extremely slow: every call to
calibrate_cyclecounter() takes about 1 second.

Instead, let's only call calibrate_cyclecounter() once, on the primary CPU.
Multiprocessor i386 machines will now boot much faster.

ok deraadt@