x86: distinguish CPU offlining from CPU removal

x86: distinguish CPU offlining from CPU removal

In order to be able to service #MC on offlined CPUs, the GDT, IDT,
stack, and per-CPU data (which includes the TSS) need to be kept
allocated. They should only be freed upon CPU removal (which we
currently don't support, so some code is becoming effectively dead for
the moment).

Note that for now park_offline_cpus doesn't get set to true anywhere -
this is going to be the subject of a subsequent patch.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>