Changes

tests/backends.c: fix a failure when /sys isn't available on Linux on non-x86

Since 2.9, Linux discovery aborts if /sys isn't available.
Usually it will fallback to the x86 backend... except on non-x86 platforms obviously
where it would fallback to "noos" instead.

The hwloc_backends.c test verified that the "Backend" info attribute was the same
in the XML-loaded topology and the original topology, but it did not handle the
case where that info attribute did not exist at all, which is the case when
only "noos" is used.

This is first reported by Simon South on https://issues.guix.gnu.org/61493
Thanks to Ludovic Courtes for forwarding to me.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

hwloc-gather-cpuid: update some comments

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

hwloc-gather-cpuid: add 0x80000023

Available at least since AMD Zen3.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

hwloc-gather-cpuid: add AMD 0x80000026 extended topology leaf

Likely available in Zen4.

We now cover AMD CPUID of Fam19h Model11h PPR from 2022/10.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>