tests/docker: do not duplicate rules for hexagon-cross

tests/docker: do not duplicate rules for hexagon-cross

The commands for docker-image-debian-hexagon-cross are the same as those
in debian-toolchain-run, just with a nonstandard path to build-toolchain.sh.
Reuse the definition by renaming the debian-hexagon-cross.docker.d directory.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-9-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-12-alex.bennee@linaro.org>