os/sys: populate richer RSysCpu / RSysNode topology attributes

[haaspors]

r_sys_cpu_discover (rlib/os/rsys.c:585) builds each RSysCpu with
only idx and node set, leaving /* TODO! */ (:595) — no
core/package id, cache hierarchy, frequency or sibling/SMT information
is captured. Flesh out the per-CPU (and, where relevant, per-node)
attributes so callers can reason about the topology beyond bare counts.

Minor related fallback: r_sys_cpuset_for_node (:373) is implemented
for win32 / Linux / macOS but its #else (unknown-OS) branch is a
no-op; either provide a generic fallback or leave it documented as
unsupported.

Related: NUMA topology memory gap in #229, and the NUMA allocation API
in #17.