tlm: Initial import of tlm/gem5 bridge code.

This is a slightly mangled version of the existing bridge code in
util/tlm/src/. The changes fix some small style issues, change to gem5
specific include paths, and removes the Gem5SimControl code. That code
coordinates gem5 with the external systemc kernel, and in this usage
there's no external kernel.

The code imported here compiles, but it isn't yet expected to work.

Change-Id: I9c593a52e2554534720d21cd31a03e543ad897ad
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17231
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>