systemc: Implement sc_export.

systemc: Implement sc_export.

This change ignores the rule that sc_exports all have to be bound
exactly once and only by the end of elaboration. If it's bound more
than once, then the earlier binding will be overwritten, and if it's
not bound at all then it will act like a null pointer. To accomodate
doing those checks in the future, the sc_export_base constructor and
destructor are in the .cc file even though they do very little so that
they can be extended to track a list of all exports which exist.

Change-Id: Ie9a3416b8fa87bca55bc9f87f3238c4de3c2e729
Reviewed-on: https://gem5-review.googlesource.com/12079
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>