EclipseFileManager doesn't properly handle -extdirs for target 8

[iloveeclipse]

Found during #2551.

org.eclipse.jdt.internal.compiler.tool.EclipseFileManager.handleOption(String, Iterator<String>) doesn't properly handle -extdirs argument (see added TODO).

The condition this.isOnJvm9 checks whether we run on Java 9 and the code will throw an error if -extdirs is given, however, if I understand it right, -extdirs is allowed with -target == 8 independently on which JVM we compile.