This adds hyper-threading information to /proc/cpuinfo, if relevant: the
physical processor id and the number of sibling units in this core.

The naming of the fields were debated a bit on lkml and the names below
offend the least number of people, do not break glibc, and are the same
as those in 2.4-ac.

This is in 2.4-ac, 2.5-mm, and vendor kernels from RedHat, SuSE, etc.