build_rootfs.sh: Add support for arm64

Support building 'foreign' architecture images for wheezy and stretch.

Because the other control files were already architecture agnostic, this
change is very simple.

Bug: 118446450
Test: ./build_rootfs.sh -a arm64
Change-Id: Ib62d1ed319224dd3d4861172fed998bb42ab7280
Signed-off-by: Alistair Strachan <astrachan@google.com>