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: 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>