systemc: Promote some functions to sc_export_base.

systemc: Promote some functions to sc_export_base.

There are a few functions which return a generic sc_interface pointer
which were (in the spec) defined to be in the interface type specific
sc_export class. They don't need to be and aren't in the Accellera
implementation, and without having them in the base class there's no
good way to get at a generic interface pointer from an export.

Change-Id: Iba692c79bf1d4f7684f28447d8b22c88ef4b804d
Reviewed-on: https://gem5-review.googlesource.com/12043
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>