website: Add new ubuntu-18.04 arm64 disk image

This is adding the new ubuntu-18.04 arm64 disk image reference to
guest_binaries page

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: I7ca78f930f597a64ccab75e09f3e5f93b101b201
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/37118
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed