python: Fix debug flag listing regression

python: Fix debug flag listing regression

The PyBind11 changes slightly modified gem5's internal debug
interfaces. The corresponding change to the public API went missing
before the new bindings were merged. This change updates the Python
glue to use the new interface.

Change-Id: I3ecca5a3f6c35b99d55126d697371124f81a12dd
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Matteo Andreozzi <matteo.andreozzi@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3140
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>