mem, sim, systemc: Reorganize Port and co.s bind, unbind slightly.

The base Port class can keep track of its peer, and also whether it's
connected. This is partially delegated away from the port subclasses
which still keep track of a cast version of their peer pointer for
their own conveneince, so that it can be used by generic code. Even
with the Port mechanism's new flexibility, each port still has
exactly one peer and is either connected or not based on whether there
is a peer currently.

Change-Id: Id3228617dd1604d196814254a1aadeac5ade7cde
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20232
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Provide Port wrapper classes for sc_port

Add Port wrappers for sc_port, sc_interface, and sc_export.

Change-Id: Ib70cb7466461c8d32e3a260bf2a2d60a6ded87c3
Signed-off-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19788
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>