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>

ruby: garnet2.0
Revamped version of garnet with more optimized single-cycle routers,
more configurability, and cleaner code.

config: add port directions and per-router delay in topology.
This patch adds port direction names to the links during topology
creation, which can be used for better printed names for the links
or for users to code up their own adaptive routing algorithms.
It also adds support for every router to have an independent latency
value to support heterogeneous topologies with the subsequent
garnet2.0 patch.

Ruby: use ClockedObject in Consumer class
Many Ruby structures inherit from the Consumer, which is used for scheduling
events. The Consumer used to relay on an Event Manager for scheduling events
and on g_system_ptr for time. With this patch, the Consumer will now use a
ClockedObject to schedule events and to query for current time. This resulted
in several structures being converted from SimObjects to ClockedObjects. Also,
the MessageBuffer class now requires a pointer to a ClockedObject so as to
query for time.

sim: Include object header files in SWIG interfaces

When casting objects in the generated SWIG interfaces, SWIG uses
classical C-style casts ( (Foo *)bar; ). In some cases, this can
degenerate into the equivalent of a reinterpret_cast (mainly if only a
forward declaration of the type is available). This usually works for
most compilers, but it is known to break if multiple inheritance is
used anywhere in the object hierarchy.

This patch introduces the cxx_header attribute to Python SimObject
definitions, which should be used to specify a header to include in
the SWIG interface. The header should include the declaration of the
wrapped object. We currently don't enforce header the use of the
header attribute, but a warning will be generated for objects that do
not use it.

network: convert links & switches to first class C++ SimObjects

This patch converts links and switches from second class simobjects that were
virtually ignored by the networks (both simple and Garnet) to first class
simobjects that directly correspond to c++ ojbects manipulated by the
topology and network classes. This is especially true for Garnet, where the
links and switches directly correspond to specific C++ objects.

By making this change, many aspects of the Topology class were simplified.