scons,ruby: do not generate unnecessary files

scons,ruby: do not generate unnecessary files

Do not generate garnet tester file or Ruby debug headers without a Ruby
protocol (i.e. PROTOCOL=None). It makes no sense to include these files
into the build when there will be no protocol to utilize them.

Change-Id: I8db4dd532f60008217a10c88a2e089f85df9d104
Reviewed-on: https://gem5-review.googlesource.com/8381
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>