Add self-IPI support

We were wrongly interpreting the destination field on self-IPIs which
caused misdirected IPIs and sometimes also false warnings about cell
boundary violations.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Add memory barrier before signaling CPU init completion

We get this barrier by accident via previous printk and its output
spinlock, but better do not rely on this.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>