python: Don't assume SimObjects live in the global namespace

The importer in Python 3 doesn't like the way we import SimObjects
from the global namespace. Convert the existing SimObject declarations
to import from m5.objects. As a side-effect, this makes these files
consistent with configuration files.

Change-Id: I11153502b430822130722839e1fa767b82a027aa
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15981
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

cpu: Make checker CPUs inherit from CheckerCPU in the Python hierarchy

Checker CPUs currently don't inherit from the CheckerCPU in the Python
object hierarchy. This has two consequences:
* It makes CPU model discovery from the Python world somewhat
complicated as there is no way of testing if a CPU is a checker.
* Parameters are duplicated in the checker configuration
specification.

This changeset makes all checker CPUs inherit from the base checker
CPU class.

cpu: Add header files for checker CPUs

In order to create reliable SWIG wrappers, we need to include the
declaration of the wrapped class in the SWIG file. Previously, we
didn't expose the declaration of checker CPUs. This patch adds header
files for such CPUs and include them in the SWIG wrapper.

CheckerCPU: Re-factor CheckerCPU to be compatible with current gem5

Brings the CheckerCPU back to life to allow FS and SE checking of the
O3CPU. These changes have only been tested with the ARM ISA. Other
ISAs potentially require modification.