Fix intr_machdep.c for ARM64

On ARMv8 IPIs are mapped to 0-15. Incrementing the number by 16
is wrong, because it sets a reserved bit in the IPI register.
This patch removes all "+16" to comply with specs.

Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3029