rusty: Account for disabled but offline CPUs

[Byte-Lab]

As described in https://bugzilla.kernel.org/show_bug.cgi?id=218109,
#147 and sched-ext/sched_ext#69, AMD chips can
sometimes report fully disabled CPUs as offline, which causes us to
count them when looking at /sys/devices/system/cpu/possible.

Additionally, systems can have holes in their active CPU maps. For
example, a system with CPUs 0, 1, 2, 3 possible, may have only 0 and 2
active. To address this, we need to do a few things:

1. Update topology.rs to be clear that it's returning the number of
   possible CPUs in the system. Also update Topology to only record
   online CPUs when creating its span and iterating over sysfs when
   creating domains. It was previously trying to record when a CPU was
   online, but this was actually broken as the topology directory isn't
   present in sysfs when the CPU is offline.

2. Schedulers should not be relying on nr_possible_cpus for anything
   other than interacting with per-CPU data (e.g. for stats extraction),
   or e.g. verifying maximum sizes of statically sized arrays in BPF. It
   should not be used for e.g. performing load calculations, etc. With
   that said, we'll also need to update schedulers to not rely on the
   nr_possible_cpus figure being exported by the topology crate. We do
   that for rusty in this patch, but don't fix any of the others other
   than updating how they call topology.rs.

3. Account for the fact that LLC IDs may be non-contiguous. For example,
   if there is a single core in an LLC, then if we assign LLC IDs to
   domains, then the domain IDs won't be contiguous. This doesn't fit
   our current model which is used by e.g. infeasible_weights.rs. We'll
   update some of the code in rusty to accomodate this, but we'll need
   to do more.

4. Update schedulers to properly reset themselves in the event of a
   hotplug event. We'll take care of that in a follow-on change.

[arighi]

LGTM, thanks for updating also rustland and rlfifo!