hwpmc: simplify arm64 kernel stack unwinding

Use the unwind_frame() function, which properly validates the frame
pointer and uses ADDR_MAKE_CANONICAL() for the pc, required when PAC is
enabled.

Reviewed by: andrew, markj, jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39934