Relativize java classpaths relative to the build root

JIs original relativization code didn't work because java was executed with subprocess.call, and this call was given a different cwd argument (the build root) than the user's actual current working directory. I have made the classpaths relativized relative to the build root as opposed to the current working directory, and this seems to work.

Also added classpath length logging (so we can know when we are getting close to the maximum argument length).

Testing Done:
./build-support/bin/ci.sh all tests passed

Reviewed at https://rbcommons.com/s/twitter/r/516/