numa: discover NUMA topology from ACPI SRAT/SLIT

[gburd]

What

OSv has no notion of NUMA: the scheduler and allocator treat memory as flat,
which leaves performance on the table on multi-socket bare-metal (the large
EC2/GCP metal instances that are a target for Postgres-over-ZFS workloads).

This is the first, discovery-only step of NUMA support. It adds a numa::
module that parses the ACPI SRAT (System Resource Affinity Table) and SLIT
(System Locality Distance Information Table) at boot, right after
acpi::init(), and exposes the topology.

API

• numa::nr_nodes() / numa::available()
• numa::node_of_cpu(cpu_id) -- resolved by correlating SRAT APIC ids with the
  APIC ids the MADT parse already recorded on each sched::cpu
• numa::distance(from, to) -- from SLIT (10 == local, per ACPI convention),
  defaulting to 10 local / 20 remote when no SLIT is present
• numa::memory_ranges() -- physical ranges tagged with their node

It handles the SRAT CPU-affinity, x2APIC CPU-affinity and memory-affinity
subtables, and only trusts SLIT if its locality count matches the node count
SRAT reported. On a machine with no SRAT (the common single-node VM) it reports
one flat node and available() == false.

Scope

This intentionally does not yet change allocation or scheduling; it only
makes the topology available so later work (a node-aware allocator, scheduler
affinity, mbind/get_mempolicy) can build on it. Nothing changes behavior on
existing single-node setups.

Testing

tests/tst-numa.cc validates the invariants (>= 1 node, every CPU maps in
range, distance diagonal == 10 and off-diagonal >= 10, memory ranges name valid
nodes). Verified on OSv under KVM both without NUMA (reports 1 flat node) and
with a QEMU 2-node -numa config (reports 2 nodes, 3 memory ranges,
available).

[gburd]

Superseded by #1437 (rebased onto current master; GitHub auto-closed this one when the branch was updated). Please review #1437 instead.