systemc: Fix TLM related includes.

There are a couple things this CL fixes related to the TLM #includes.

1. Removes #includes of <systemc> and <tlm>. These bring in a header
file from boost which shouldn't be necessary but which some of the
tests (and likely some external code) depends on. We avoid including
those in files built into gem5 itself so that gem5 isn't dependent on
boost.

2. All includes in ext should be relative. That way those headers can
be removed from gem5 and still build, allowing them to be moved over
to or referenced from a foreign codebase which isn't part of gem5.

Change-Id: I76e267385b48cb4fe93aea89ec8319c76465a0a4
Reviewed-on: https://gem5-review.googlesource.com/c/15796
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Replace sc_core::sc_type_index with std::type_index.

The former is either the same as the later, or a custom wrapper if
C++11 isn't supported. Since we generally expect C++11 support, we can
skip the indirection.

Change-Id: I9a45e3854bb7cc56d094e3fe2773fe4b5c94403b
Reviewed-on: https://gem5-review.googlesource.com/c/15297
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

systemc: Rename tlm .cpp files to .cc and add SConscripts.

Change-Id: Ia30192b441dd34bc7165c6335386c88c0015fdf6
Reviewed-on: https://gem5-review.googlesource.com/c/15295
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>