systemc: Teach the TLM bridges how to use gem5's new backdoor mechanism.

This change teaches the TLM bridges to translate between TLM's DMI
mechanism and gem5's backdoor mechanism.

Change-Id: I942a6cce4fb87f10e8173f4ee49b6c7b0ffa7e4a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17591
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Templatize the gem5/TLM bridge SimObjects.

The C++ side is templated, and there are python versions for each
(currently two) width of bridge supported.

Change-Id: I4baa9f22d4c87629d45e9e1292eb66c65d25a655
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17234
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Create unified gem5/TLM bridge SimObjects.

These objects expose a standard TLM initiator or target socket with
width 64, and a gem5 slave or master port. What goes in one type of
port comes out the other with the appropriate conversion applied.

Change-Id: I65e07f746d46d3db0197968b78fffc5ddaede9bf
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17232
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>