run.javac-1.8 mode

Change-Id: Ic7a9491776997395dd262c255c40d01b8bf20b47
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/180082
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Stephan Herrmann <stephan.herrmann@berlin.de>