Support benchmarking Scala versions built with compiler flags

[lrytz]

Currently a Scala version is a commit sha. We'd like to be able to run the benchmarks for a Scala version built with different compiler flags.

• Internally, a ScalaVersion already is (sha: String, compilerOptions: List[String]) - Nil means the default, i.e. -opt:l:classpath in 2.12
• The options are currently unused, the integrate-bootstrap jobs don't allow passing them in
• Need a naming scheme for the build artifacts - either build it from the compilerOptions or require users to provide it