x86: use wait_for_init_deassert in x86_64

wraps the busy loop for wait_for_init_deasserted() in a function,
so smp_callin in x86_64 looks like more i386

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