systemc: Get rid of leftovers from unimplemented warnings past.

These warnings were removed when the functionality they warned about
was implemented, but there were some leftovers like unnecessary
includes and some helper functions which hid gem5 specific headers
from the ext directory.

Change-Id: Ic886ac0f1264687524e3a7b7eaab8836f318a5a2
Reviewed-on: https://gem5-review.googlesource.com/c/13398
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Switch to using predefined messages for core.

Create and use predefined messages for core which match the ones
Accellera uses.

Change-Id: I05b1398933f753946d5917f39d0f39c7cb45ed9f
Reviewed-on: https://gem5-review.googlesource.com/c/13323
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add some additional error checks.

Change-Id: I19c5e6f1795c2777dbe7d210cfa01f6ced2020f3
Reviewed-on: https://gem5-review.googlesource.com/c/12815
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement most of sc_object.

To avoid making it hard to change sc_object's implementation in the
future, this change keeps most of the data members out of sc_object and
keeps them in a seperate Object which is managed independently but
still matches to the sc_objects one to one.

This change also moves away from the SystemC/sc_gem5 namespace pair in
favor of sc_gem5. Having two namespaces with classes, etc, living in
both was complicating things. Having to use a namespace that doesn't
fit in one scheme or the other isn't great, but it's the lesser of two
evils.

Change-Id: Ib59c3c515ca98c7fe519c59e9fe9270304b71cc0
Reviewed-on: https://gem5-review.googlesource.com/11611
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Construct and manage a module name stack.

Change-Id: I5f7f64d6c3d7e08ec6d2529f3c5d84fbfc2c421b
Reviewed-on: https://gem5-review.googlesource.com/10850
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>