Start to clean up arm64 address space selection

On arm64 we should use bit 55 of the address to decide if aan address
is a user or kernel address. Add a new macro with this check and a
second to ensure the address is in teh canonical form, i.e.
the top bits are all zero or all one.

This will help with supporting future cpu features, including Top
Byte Ignore, Pointer Authentication, and Memory Tagging.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31179