arm64: Remove struct arm64_frame

It was used in one place and was added specifically to support dtrace
stack unwinding code. Write an equivalent expression using struct
unwind_state instead. No functional change intended.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40538

(cherry picked from commit d3251842321e7a5ac8bbb2aec128328cd10f3a84)