alpha: bound EV6 logout decoding by the processor area

The ES40 Service Guide, Table D-19, documents a correctable frame with
its system area at 0x58 and total size 0x80. The existing EV6 decoder
interprets system words as extended CPU registers and reads past its end.
This establishes the layout for ES40; whether Titan and Nautilus PALcode
also emits this short frame has not been verified.

Validate the processor area before decoding it and only print extended
registers when present. Preserve rejected frames with a raw dump rather
than trusting their invalid offsets. Cap the dump at 1 KiB and round down
to complete quadwords. This still relies on firmware supplying an
accessible frame of its advertised size; the cap is not pointer validation.

Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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