util: Updated Dockfiles with the libelf-dev dep

util: Updated Dockfiles with the libelf-dev dep

This is required if we eventually remove `ext/libelf` (
https://gem5.atlassian.net/browse/GEM5-752), otherwise our tests will
fail.

The corresponding Docker images have been built and uploaded to:
https://gcr.io/gem5-test/

Change-Id: I1bd069dfb968b56eac4c4da33929b5ff895eaa6f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33596
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>