x86: use create_idle struct in do_boot_cpu

Use a new worker, with help of the create_idle struct
to fork the idle thread. We now have two workers, the first
of them triggered by __smp_prepare_cpu. But the later is
going away soon.

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