Disable promotion on pcpu memory on arm64

We need to be careful to not promote or demote the memory containing
the per-CPU structures as the exception handlers will dereference it
so any time it's invalid may cause recursive exceptions.

Add a new pmap function to set a flag in the pte marking memory that
cannot be promoted or demoted and use it to mark pcpu memory.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35434