systemc: Add a nonstandard sc_status pretty printer operator.

This operator exists in the Accellera implementation, and is necessary
to make the test output match.

Change-Id: I266629d6c936d4846e88e35af36555fb392b181c
Reviewed-on: https://gem5-review.googlesource.com/12074
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add non-standard sc_status constants.

Change-Id: I02f52df318473906062035b188348fb875daad08
Reviewed-on: https://gem5-review.googlesource.com/11276
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add some missing functions which interact with the scheduler.

Change-Id: Ifc8c8d4a7bb6e941485e80f4884cfa4bb648c17c
Reviewed-on: https://gem5-review.googlesource.com/10846
Reviewed-by: Jason Lowe-Power <jason@lowepower.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>