scons: Turn the ProtoBuf and GrpcProtoBuf classes into methods.
scons: Turn the ProtoBuf and GrpcProtoBuf classes into methods.

These now instantiate Builders for the protobuf .cc and .h files of the
right flavors, and install the .cc files as sources.

Also, this sets up the builder for .cc files so that it will know how to
build those files from .proto using the protobuf compiler. This is
optional, but will make it possible to pass .proto files in place of .cc
files or even object files as sources for other builders, and for scons
to figure out what chain of rules to use to get the desired results.

This is a step towards handing over more responsibility to scons and
eliminating at least some of our bespoke build code.

Change-Id: I7188a0917e999ad9148f7079830b2c26bcd563db
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48134
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5