Teach the arm64 kernel to identify the Arm AEM

The Arm Architecture Envelope Model is a simulator that models the
architecture rather than any specific implementation. Add its part ID
macro and add it to the list of Arm CPUs we can decode.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 2531f067ea0e9c77b081445800de8e9584d7d4ab)