systemc: Use an std::list to track all modules.

systemc: Use an std::list to track all modules.

This is less efficient when modules are destroyed since the list isn't
sorted, and each module needs to find its own entry to remove. The
benefit is that entries added to the end of the list while the list is
being iterated over will still be included, and that the order the
modules are added will be preserved so that it matches what the order
in the regression tests.

Change-Id: I5af5d15f316fa58561e8fd9ca77f667ddc8b2c5e
Reviewed-on: https://gem5-review.googlesource.com/12077
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>