arm64: Enable MOPS in userspace

Detect presence of FEAT_MOPS, and enable instruction set and set HWCAP2 flag
if present.

Add handler for MOE exceptions.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54558

(cherry picked from commit 591c7a08bf8addce4b047ef9c8033c33099e4688)