arm64: Return all registers to gdb when able

When the kdb thread is the current thread we read the registers from
the trap frame. As this contains all general purpose registers we can
use it to read these in the gdb stub. This allows us to include the
non-callee saved registers, e.g. function arguments.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44360