mem: Clarify unit of DRAM controller buffer size
arm: Fixes based on UBSan and static analysis

Another churn to clean up undefined behaviour, mostly ARM, but some
parts also touching the generic part of the code base.

Most of the fixes are simply ensuring that proper intialisation. One
of the more subtle changes is the return type of the sign-extension,
which is changed to uint64_t. This is to avoid shifting negative
values (undefined behaviour) in the ISA code.
config: Fix checkpoint restore in C++ config example

This patch fixes the checkpoint restore option in the example of C++
configuration (util/cxx_config).

The fix introduces a call to config_manager->startup() (which calls startup
on all SimObjects managed by that manager) to replicate the loop of
SimObject::startup calls in src/python/m5/simulate.py::simulate guarded by
need_startup.  As util/cxx_config/main.cc is a C++ analogue of
src/python/mt/simulate.py, it should make a similar set of calls.