First of all in case when the number CPUs change
prevent out-of-bound index access in
waybar::modules::CpuUsage::getCpuUsage()
Secondly on Linux when updating CPU usage
read /sys/devices/system/cpu/present
and use it to detect the offline CPUs missing from /proc/stat
For offline CPUs report 0 usage and "offline" in the tooltip
Fixes issue #3498
On Linux one can test this functionality with:
echo 0 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu1/online
On non-Linux OSes I'm not sure how to detect offline CPUs,
so I didn't add the offline CPU detection there
but at least CPU number change should not cause a crash there anymore
or cause memory safety issues after this fix
