x86: do not zap_low_mappings in __smp_prepare_cpus

It was okay when cpus were cold booted before this point.
But with the new state machine, they will not have arrived to
the trampoline yet. zapping low mappings will have the bad effect
of breaking it completely after paging enablement

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