arm/arm64: smp: assert secondaries only boot once

It's tempting to try running smp_run() more than once, but that
won't currently work. In the meantime, let's fail better. Also,
there's no reason to expose the structure in smp.h

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20170529135804.22891-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>