arm64: don't pass user trapframe to kdb_trap()

This effectively undoes the changes made in r321571. While useful, it is
inconsistent with how other architectures pass trapframes to kdb. This
change is also required to get a working gdb(4) stub on arm64, as
otherwise the backtrace will begin too early.

As of 088a7eef95b1, this information can still be obtained via
"show registers/u".

Reviewed by: jhb (slightly earlier version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Pull Request: https://reviews.freebsd.org/D27719