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: Seperate the "external" header interface.

Most (but not all) of the SystemC headers are part of the "external"
interface that an existing, standard compliant module would include
through <systemc.h> or <systemc>. Since those follow slightly different
rules (relative includes, no gem5 includes), this change separates them
out so that they're easier to identify.

Also, this change moves the other files into a "core" subdirectory,
with the intention to add a "dt", aka data type, directory some time in
the future when those standard defined types are implemented.

Change-Id: Ida63f9cc0bc0431024d4dd691cc5b22b944a99a8
Reviewed-on: https://gem5-review.googlesource.com/10835
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>