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: Change how the default timescale is set.

The spec says that the default timescale should be 1 PS, but the
Accellera implementation uses the time resolution.

Change-Id: I7b307a33ef0856e9c19d81e401b15691275d4978
Reviewed-on: https://gem5-review.googlesource.com/c/12975
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Print an Info when setting a tracefile's time unit.

This matches Accellera's output for the tests.

Change-Id: Ie67fd673a980b8ca2fdcd8f357ff7993a9091221
Reviewed-on: https://gem5-review.googlesource.com/c/12972
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement general and VCD trace support.

This doesn't include WIF trace support, but does make allowances for
adding it in the future.

Change-Id: Ifb62f40a7d8a13e94463930a44ac4b1cf41e3009
Reviewed-on: https://gem5-review.googlesource.com/c/12826
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>