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: Replace the fastmem with a new CPU model

The AtomicSimpleCPU used to be able to access memory directly to speed
up simulation if no caches are used. This is fine as long as no
switching between CPU models is required. In order to switch to a new
CPU model that requires caches, we currently need to checkpoint the
system and restore it into a new configuration. The new
'atomic_noncaching' memory mode provides a solution that avoids this
issue since caches are bypassed in this mode. This changeset removes
the old fastmem option from the AtomicSimpleCPU and introduces a new
CPU, NonCachingSimpleCPU, which derives from the AtomicSimpleCPU.

The NonCachingSimpleCPU uses the same mechanism as the AtomicSimpleCPU
used to use when accessing memory in when fastmem was enabled.

This changeset also introduces a new switcheroo test that tests
switching between a NonCachingSimpleCPU and a TimingSimpleCPU with
caches.

Change-Id: If01893f9b37528b14f530c11ce6f53c097582c21
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12419
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>