python: Add support for introspecting scalar stats

python: Add support for introspecting scalar stats

This change adds a wrapper for the ScalarInfo stat type to enable
introspection of scalar stats from Python. Due to the slightly
confusing use of proxy objects in the stat system, PyBind11 fails to
automatically cast to the right wrapper type. This is worked around in
the by explicitly casting to the relevant type's Python wrapper.

To make the interface more Python-friendly, this change also changes
the semantics of resolveStat to raise an exception if the stat can't
be found.

Change-Id: If1fc6fe238fc9d69d4e22369a4988a06407d2f7c
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33176
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>