arm64: smp: Fix broadcast_ipi issue

This commit mainly fixes the broadcast_ipi issue when one core broadcast
ipi to other cores using gic_raise_sgi. The issue doesn't affect the
functionality of Zephyr SMP but will happen when Zephyr runs on Xen.
Suppose Xen provides 4 CPUs to the Zephyr guest, for example, when cpu0
broadcasts ipi to the rest of the cores, the mask should be 0xE(0b1110),
but for now, Zephyr will send 0xFFFE. So for Xen, it will receive a
target list containing many invalid CPUs which don't exist. My solution
is: to generate the target list according to the online CPUs.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>
(cherry picked from commit 3122b9ed)