gitlab-ci: Use separate docker images for cross builds

Using x86_build-base as a separate base layer as well for storage &
network bandwidth efficiency.

Using separate images allows dropping the workarounds from the cross
build job scripts.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5296>
7 files changed