Add definitions and support for the AMD k6-2, Pentium MMX (i586/MMX),
and Pentium II, III and IV processors (p2, p3, p4), as well as 'mmx' and
'3dnow' MACHINE_CPU tags as appropriate. In the near future this will
be used to control various ports which have MMX/3dNow optimizations,
instead of the ad-hoc methods currently used.

Reviewed by: peter