arm64: Enable EPAN and IESB

These are both safe to enable. EPAN only affects PAN when it's enabled,
and IESB makes exception entry and exit an Implicit Error
Synchronization event.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51013

(cherry picked from commit 68b3d0cbe03467abac0523bbeea0cfc34efd9b05)