Fine tune the code that decides when to use CPUID. For now, use it

Fine tune the code that decides when to use CPUID.  For now, use it
only for Athlon model 1, 2, 3, or 4 processors.  According to AMD, the
model 6 is OK, and perhaps the model 7 as well.  The model 4 is known
to be bad, and models 1, 2, and 3 are probably bad too.  No Intel
processors are known to be bad.