Switch to sparc64 to debian:stretch

Looks like ubuntu 17.04 is broken now and it fails to link on 17.10 and 18.04.
It seems to work on stretch though!

ci/docker/sparc64-unknown-linux-gnu/Dockerfile

-# link fails on 17.10
-FROM ubuntu:17.04
+FROM debian:stretch
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         curl ca-certificates \