misc: Added Dockerfile for clang

misc: Added Dockerfile for clang

This will create a Docker image with all gem5 dependencies, allowing for
a specific clang version to be specified via `--build args version=X`.
I.e., to create an image with clang 6,
`docker build util/dockerfiles/clang-version --build-arg version=6`

Issue-on: https://gem5.atlassian.net/browse/GEM5-235
Change-Id: I7d12acd265d7aef2a9e90f348f4214231effe509
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26484
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.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>