sim: Rename the SimObject cxx_bases field to cxx_extra_bases.

sim: Rename the SimObject cxx_bases field to cxx_extra_bases.

cxx_bases adds in additional c++ base classes beyond those implied by
the python SimObject inheritance hierarchy. To imply the fact that
these are additional bases, and to disambiguate a future mechanism
which changes the implied bases, this flag/field is being renamed from
cxx_bases to cxx_extra_bases.

As far as I can tell, this field was only used internally in
SimObject.py.

Change-Id: Ie7cc3d0107ff71cc31424d6e20c9a2f430022ab9
Reviewed-on: https://gem5-review.googlesource.com/10661
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>