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>
3 files changed