Disable promotion on pcpu memory on arm64

[andrew]

Summary

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.

Test Plan

Alternative to D35084