smp: Use bitwise operation to count cpu number

[aokblast]

Summary

Previously, we iterated over all CPUs using CPU_FOREACH and checked
individual bits to count valid CPUs. Refactor this to use a bitwise AND
and popcount to count the number of enabled bits directly.

[aokblast]

Thanks! Thought I sucessfully booted up on a AMD64 machine, this patch touches the basic logic of smp so I will think twice before landing it.

[markj]

In D54474#1244841, @aokblast wrote:

Thanks! Thought I sucessfully booted up on a AMD64 machine, this patch touches the basic logic of smp so I will think twice before landing it.

It's probably ok, but yes it's a good idea to keep patches in a local tree that you test often. Then you can build some confidence in the changes before landing them.