mem: Make PortProxy use a delegate for a sendFunctional function.

mem: Make PortProxy use a delegate for a sendFunctional function.

The only part of the MaserPort the PortProxy uses is the sendFunctional
function which is part of the functional protocol. Rather than require
a MasterPort which comes along with a lot of other mechanisms, this
change slightly adjusts the PortProxy to only require that function
through the use of a delegate. That allows lots of flexibility in how
the actual packet gets sent and what sends it.

In cases where code constructs a PortProxy and passes its constructor
an unbound MasterPort, the PortProxy will create a delegate to the
sendFunctional method on its own.

This should also make it easier for objects which don't have
traditional gem5 style ports, for instance systemc models, to implement
just the little bit of the protocol they need, rather than having to
stub out a whole port class, most of which will be ignored.

Change-Id: I234b42ce050f12313b551a61736186ddf2c9e2c7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20229
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>