systemc: Add systemc and systemc.h header files.

systemc: Add systemc and systemc.h header files.

These are the header files as defined by the standard, although some
predefined channel types and most of the sc_dt namespace have yet to be
stubbed out, and so those portions are excluded.

Change-Id: Ic70f887c06e591974a4265c820eb0fdfa740d19a
Reviewed-on: https://gem5-review.googlesource.com/10838
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>