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

[mhorne]

Summary

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

With D27705, this information can still be obtained via show registers/u.

[andrew]

sys/arm64/arm64/trap.c

423

You need to remove this so the next instruction is executed

[mhorne]

Remove the tf_elr manipulation.

[jrtc27]

sys/arm64/arm64/trap.c

423

Is it not now going to just return and immediately re-trap on the same breakpoint instruction?

[mhorne]

sys/arm64/arm64/trap.c

423

For ddb, tf_elr is advanced by BKPT_SKIP in db_stop_at_pc. For gdb, the client advances the PC.

Previously this would have been applied to td->td_frame, not frame, hence the need for the manual increment here.

[jrtc27]

sys/arm64/arm64/trap.c

423

Ok thanks