[SPARK-15451][BUILD] Use jdk7's rt.jar when available.

This helps with preventing jdk8-specific calls being checked in,
because PR builders are running the compiler with the wrong settings.

If the JAVA_7_HOME env variable is set, assume it points at
a jdk7 and use its rt.jar when invoking javac. For zinc, just run
it with jdk7, and disable it when building jdk8-specific code.

A big note for sbt usage: adding the bootstrap options forces sbt
to fork the compiler, and that disables incremental compilation.
That means that it's really not convenient to use for normal
development, but should be ok for automated builds.

Tested with JAVA_HOME=jdk8 and JAVA_7_HOME=jdk7:
- mvn + zinc
- mvn sans zinc
- sbt

Verified that in all cases, jdk8-specific library calls fail to
compile.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #13272 from vanzin/SPARK-15451.