systemc: Add a simple implementation for sc_mempool.

This class is supposed to be a more efficient way to manage small bits
of memory. At least for now, new and delete will do the same job
functionally. Also, the heap manager may be just as efficient as
whatever custom mechanism sc_mempool would use.

Change-Id: I6fdc01a69ca017d94b14a15a196ad29b66ef9858
Reviewed-on: https://gem5-review.googlesource.com/c/13292
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

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>