systemc: Ensure SC_LONG_64 is defined and available.

The datatype code was checking if SC_LONG_64 was defined to determine
if a long was 64 bits. The code that would define that value was
dropped when porting over from the Accellera implementation, and so
the wrong code was being included. This change both makes those checks
look at the *value* of SC_LONG_64 to ensure that it's not missing by
accident, and assigns it a value in sc_fxdefs.hh.

Change-Id: Ie9bb1146452a3db1d9d99c0db575098bb06463ff
Reviewed-on: https://gem5-review.googlesource.com/c/12616
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

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>