Add experimental 16k page support on arm64

Add initial 16k page support on arm64. It is considered experimental,
with no guarantee of compatibility with a userspace or kernel modules
built with the current a 4k page size as code will likely try to pass
in a too small size when working with APIs that take a multiple of a
page, e.g. mmap.

As this is experimental, and because userspace and the kernel need to
have the PAGE_SIZE macro kept in sync there is no kernel option to
enable this. To test a new image should be built with the
PAGE_{SIZE,SHIFT,MASK} macros changed to the 16k versions.

There are currently known issues with loading modules from an old
loader as it can misalign them to load on a non-16k boundary.

Testing has shown good results in kernel workloads that allocate and
free large amounts of memory as only a quarter of the number of calls
into the VM subsystem are needed in the best case.

Reviewed by: markj
Tested by: gallatin
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34793