devel/arduino: pin to openjdk8

Dependency comms/rxtx is tied to openjdk8 and installs jars in
PREFIX/openjdk8/jre/lib/ext. This directory is gone in jdk9+, but
arduino expects it.
So pin arduino to openjdk8 also.
If anybody wants this to be supported by jdk9+, patches are welcome.

PR: 292651