scons: Convert ProtoBuf to use a scons Builder and Scanner.

scons: Convert ProtoBuf to use a scons Builder and Scanner.

There are several benefits to using a Builder. First, the action we're
executing is shared between all uses of the Builder. The number of
times this particular builder is called is small, but it should still
be a little more efficient.

Second, we can use SCons's emitter mechanism to generate the .pb.cc and
.pb.h target files in a little more general way.

Also, this change adds a Scanner for .proto files which will scan them
for imports and let SCons manage those implicit dependencies properly.
The scanner is a bit simplistic as described in a comment in the
source, but should work pretty well in practice with reasonably
formatted files, and in particular some files I'm working with that
include imports.

Change-Id: Iaf2498e61133d6f713d6ccaf199422b882c5894f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37276
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>