Linux kernel with CONFIG_BOOTPARAM_HOTPLUG_CPU0 as compile time
allow offlining CPU0. The Linux early discovery used CPU0 to detect
whether we should use /sys/bus/cpu or /sys/devices/system/cpu,
and whether new core/package_cpus are available.
If this fails, read the first online CPU number at look at it.
The overhead for the normal case (CPU0 online) is 0.

Thanks to Garrett Clay for the report.
Fixes #450

For hwloc 2.5, we should moderniez this code:
/sys/devices/system/cpu is unlikely going away because many
files were added in there since /sys/bus/cpu was added.
Kernels without sysfs cpu topology files are also too old now.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
(cherry picked from commit 7bcc273efd50536961ba16d474efca4ae163229b)