Enable IPIs on CPU 0 on arm and arm64

Not all interrupt controllers enable IPIs by default as the Arm
GIC specs make it an implementation defined option. As at least two
hypervisors have also previously masked the IPIs on boot.

As we already enable these IPIs on the non-boot CPUs it is expected
this is a safe operation.

Differential Revision: https://reviews.freebsd.org/D26975

(cherry picked from commit 2420f6aed9e355ff65377152ba977b3a5ac441d1)