tests/docker: use prebuilt toolchain for debian-hexagon-cross

The current docker image for cross compiling hexagon guests
is manually built since it takes >2 hours to build from source.

This patch:
 1. Solves the above issue by using the prebuilt clang
    toolchain hosted on CodeLinaro [1] and maintained by QUIC [2].
 2. The dockerfile is also switched from multi-stage to single stage
    build to allow the CI docker engine to reuse the layer cache.
 3. Re-enables the hexagon-cross-container job to be always run in
    CI and makes it a non-optional dependency for the
    build-user-hexagon job.

The changes for 1 & 2 together bring down the build time to
~3 minutes in GitLab CI when cache is reused and ~9 minutes
when cache cannot be reused.

[1]: https://github.com/CodeLinaro/hexagon-builder
[2]: https://github.com/quic/toolchain_for_hexagon/releases/

Signed-off-by: Mukilan Thiyagarajan <quic_mthiyaga@quicinc.com>
[AJB: also tweak MAINTAINERS, remove QEMU_JOB_ONLY_FORKS and comment]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20221219144354.11659-1-quic_mthiyaga@quicinc.com>
Message-Id: <20221221090411.1995037-6-alex.bennee@linaro.org>
diff --git a/.gitlab-ci.d/container-cross.yml b/.gitlab-ci.d/container-cross.yml
index 2d560e9..e0d75d5 100644
--- a/.gitlab-ci.d/container-cross.yml
+++ b/.gitlab-ci.d/container-cross.yml
@@ -34,31 +34,11 @@
   variables:
     NAME: debian-armhf-cross
 
-# We never want to build hexagon in the CI system and by default we
-# always want to refer to the master registry where it lives.
 hexagon-cross-container:
-  extends: .base_job_template
-  image: docker:stable
+  extends: .container_job_template
   stage: containers
   variables:
     NAME: debian-hexagon-cross
-    GIT_DEPTH: 1
-    QEMU_JOB_ONLY_FORKS: 1
-  services:
-    - docker:dind
-  before_script:
-    - export TAG="$CI_REGISTRY_IMAGE/qemu/$NAME:latest"
-    - export COMMON_TAG="$CI_REGISTRY/qemu-project/qemu/qemu/$NAME:latest"
-    - docker info
-    - docker login $CI_REGISTRY -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD"
-  script:
-    - echo "TAG:$TAG"
-    - echo "COMMON_TAG:$COMMON_TAG"
-    - docker pull $COMMON_TAG
-    - docker tag $COMMON_TAG $TAG
-    - docker push "$TAG"
-  after_script:
-    - docker logout
 
 hppa-debian-cross-container:
   extends: .container_job_template