systemc: Export the tlm::tlm_global_quantum class to python.

This way the python code can set up the global quantum without having
an sc_main function to do it.

Change-Id: I96df4dea0f1bfe9e3e86d4784bbda8f5b6b74d0b
Reviewed-on: https://gem5-review.googlesource.com/c/16503
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

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: Stop using the non-standard sc_time % in TLM.

The TLM headers were using the non-standard % operator on sc_time.
This change replaces that with % applied to the result of
sc_time::value().

Change-Id: Ic381eb1ada2c994d04e65896db178f58446944ae
Reviewed-on: https://gem5-review.googlesource.com/c/15062
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

systemc: Remove redundant tlm_ prefixes from file names.

We already know those files belong to tlm because of the directory
they're in. Removing the prefix makes the paths of the headers less
enormously long.

Change-Id: I869e58fae904162f353bb31f4c0919fba08dffa6
Reviewed-on: https://gem5-review.googlesource.com/c/15059
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>