Both I486_CPU and I586_CPU had been deprectated and removed from
the default GENERIC configurationbut r205336 brought them back apparently
to avoid requiring SSE in the kernel.
Removing just I486_CPU seems consistent enough at this time: we don't
support any processor without a FPU and 80486 production was stopped by
Intel on September 2007.
The PC-98 arch, which frequently involved i486 CPUs is also now gone.