scons: Add support for GRPC protobuf files.

scons: Add support for GRPC protobuf files.

These files are used to generate stubs for calling across GRPC
protocols, an RPC mechanism which is based around the protocol buffer
system.

The support for these files is heavily based on and calls into the
existing protobuf file support, but with the extra step which generates
the additional .grpc.pb.cc and .grpc.pb.h files.

Change-Id: I89022928c08aa9f7ed024b7380ddcc54ca75b55e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37277
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>