smp: add an abstraction handling detected CPUs' metadata

[wipawel]

Instead of orienting detected CPUs based on allocated percpu pages for them, use a generic and unified mechanisms to allocate and store detected CPUs metadata (including the percpu pages).

This enables better handling of the per-CPU resources and allows to easier assign tasks to specific CPUs (future commits).

This also makes the BSP identification more sane and allows to have a common API interface for finding available CPUs (and their number).