Separate location of analysis cache if we're cross compiling scala.

* Add task to determine file-name of analysis cache
* If crossPaths := true, then add scala binary version to the analysis cache name.

This makes it possible to leverage incremental compilation with the `+` command.

Fixes #1267
Merge pull request #1277 from sbt/fix/issue-1267

Separate location of analysis cache if we're cross compiling scala.