systemc: Implement the sc_attr classes.

Change-Id: Ibbe6da957b1b36687178f226e80718adc0f4ab81
Reviewed-on: https://gem5-review.googlesource.com/11609
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add some "implementation defined" stuff to sc_attr_cltn.

These "impelementation defined" methods are tested by the regression
tests, so we need to have them. We might as well have the same
general interface as the Accellera implementation since nothing there
seems overly specialized for that environment.

Change-Id: Ief6567fcd9d99d3a0f526cfa3b65043b2c828efa
Reviewed-on: https://gem5-review.googlesource.com/11185
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>