systemc: Construct and manage a module name stack.

Change-Id: I5f7f64d6c3d7e08ec6d2529f3c5d84fbfc2c421b
Reviewed-on: https://gem5-review.googlesource.com/10850
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>