systemc: Implement most of sc_inout.

Because sc_inout needs sc_dt::sc_logic and that probably calls
functions from dt_sc_mempool.cc and because those hadn't yet been
stubbed out. This change adds stubs for those as well.

Change-Id: I544a1669575b594d4612558b8b6f47668ac94414
Reviewed-on: https://gem5-review.googlesource.com/12221
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>