Add helper macros to iterate over available CPUs in the system.
CPU_FOREACH(i) iterates over the CPU IDs of all available CPUs. The
CPU_FIRST() and CPU_NEXT(i) macros can also be used to iterate over
available CPU IDs. CPU_NEXT(i) wraps around to CPU_FIRST() rather than
returning some sort of terminator.

Requested by: rwatson
Reviewed by: attilio