Systemc: Port over all of the systemc "datatype" headers.

These are the headers originally written by Accellera with a few
modifications. Most significantly, I went through and mostly (but not
entirely) manually editted them to conform to gem5 style and to be
more self consistent. Second, I resolved some macros which optionally
select features. I removed support for deprecated functions, and
otherwise enabled everything.

The actual implementation behind these headers will also be ported
over, but in a subsequent change.

Change-Id: I203d3f6c8a3af9120b946001d01defbb0643a6b6
Reviewed-on: https://gem5-review.googlesource.com/10843
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

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>