Treat cache write as a read in arm64 data faults

On arm64 we can ask the hardware to perform cache operations from
userspace. These require read permission however when the memory is
unmapped the kernel will receive a write exception. Add a check to
see if the cause of the exception is from the cache and pass a memory
read fault type to the vm subsystem.

PR: 262836
Reported by: dch
Sponsored by: The FreeBSD Foundation