dtrace/arm64: Fix user memory access routines

Use unprivileged loads to access user memory. Without this, the
accesses trap and various dtrace actions such as ustack() fail.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40540

(cherry picked from commit 6281147a963b02bb5d7b4db15ee295d6d65d0fff)