New repository for benchmark builds

[lrytz]

The benchmark tool currently uses the ordinary -integrate-bootstrap jobs which publish to scala-release-temp. We should have a repo that is less temp to keep those builds (ideally forever, if there's enough space).

We'll also need a new build job that allows passing in compiler flags and customizing the artifact name, see scala/compiler-benchq#4.

[adriaanm]

If we're going to change publishing, should also consider changing the versioning, so that sbt can derive the right binary version: scala/scala-dev#252

Let's think about how to reuse these artifacts/jobs as much as possible. It would be good to have a canonical job and long-term artifact cache that can publish once, and then be used for bisecting, benchmarking, running the community build, building nightlies,....

[SethTisue]

not sure if this should stay open since I'm not really familiar with the bench stuff