Allow ddb and dtrace use the DMAP region on arm64

When writing to memory on arm64 we may be trying to be accessing a
read-only page. In this case try to access via the DMAP region to
get a writable location.

While here simplify writing data in DDB and stop trashing the size as
it is passed into the cache handling functions.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32053