systemc: Wrap some report maps in functions.

systemc: Wrap some report maps in functions.

By declaring the map as a static variable in that function and then
returning it, we can guarantee that it's initialized relative to other
static initializers so that we don't try to use a data structure that
isn't constructed yet. This will let us get rid of the dependence on
python for setting up that mapping.

Change-Id: I031ce2039de8f5f79fbb9d76cf1363f15207b64b
Reviewed-on: https://gem5-review.googlesource.com/c/13975
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>