openjdk: Hackery to ensure that tools.jar is found

HotSpot uses the absolute path of libjvm.so to determine the java.home
property (ignoring $JAVA_HOME), which is in turn used by
ToolProvider.getSystemJavaCompiler() to load tools.jar.  So we need to
do some trickery to ensure that if java gets invoked from the jdk
output (ratherthan the jre output), it finds libjvm.so in the jdk output.