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>