x86: use specialized routine for setup per-cpu area

We use the same routing as x86_64, moved now to setup.c.
Just with a few ifdefs inside.
Note that this routing uses prefill_possible_map().
It has the very nice side effect of allowing hotplugging of
cpus that are marked as present but disabled by acpi bios.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>