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: Remove the TLM dependence on a non-standard method.

The sc_event_finder class in Accellera's implementation has a
non-standard report_error function that it uses internally. The TLM
headers were calling that function in their own event finder subclass.
This change replaces that call with what should be an equivalent bit of
code which is based on the report_error implementation.

Change-Id: Id57d26791df01403a77e70d5f4a00f650dc33599
Reviewed-on: https://gem5-review.googlesource.com/c/15063
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

systemc: Stop using the Accellera specific "none" global event object.

That event being available is nonstandard. Define our own static event
for use in that case.

Change-Id: I5e8892bd83cc1984d63949ef3e249755dca0c702
Reviewed-on: https://gem5-review.googlesource.com/c/15296
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

systemc: Rename tlm_core header files to have a .hh extentension.

This makes those files more consistent with other headers in gem5. The
top level headers (tlm and tlm.h) and the tlm_utils headers were left
alone since the user might reasonably expect those to have a well known
name. The tlm_core headers by comparison are an implementation detail,
and users shouldn't include them directly by name.

Change-Id: Ibc82e8159e47717c79d76f50ed96fdd619294a06
Reviewed-on: https://gem5-review.googlesource.com/c/15067
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>