Add a PC_IS_FULL macro to the arm64 pmap

[andrew]

Summary

This is used to handle the difference in size of pc_map between
4k and 16k pages.

[markj]

sys/arm64/arm64/pmap.c

2479

[jhb]

So I started with this locally and ended up adding some different variants. I meant to push them for review yet but haven't. In particular, the versions I have are still able to be unrolled in the case of 16k pages and by using constants can do less loads from memory. I'll upload the set of changes I have in a bit. They are also useful in CheriBSD where we have to have yet another set of constants for _N* for purecap kernels, so I tried to minimize the special cases for differing values of _N*.

[jhb]

Uploaded to D36217 and a followup change to pc_is_free in D36218.