arm64: Add a sysctl to see if features are enabled

This will also be used as a tunable to control features in a later
change.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D52357

(cherry picked from commit 9204a315a71c5aa9a9b8c11f2dcefb155dd5fc34)