gem5: add defconfig for arm64 Add a default gem5 Linux configuration for arm64 Change-Id: Ib3247dbf99b10078f023b7a393e71c573eccdd4e Signed-off-by: Geoffrey Blake <Geoffrey.Blake@arm.com> Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com> [Updated with 4.9 defaults] Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>