gdb: make get_frame_register_bytes take the next frame

Similar to the previous patches, change get_frame_register_bytes to take
the "next frame" instead of "this frame".

Change-Id: Ie8f35042bfa6e93565fcefaee71b6b3903f0fe9f
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
10 files changed