ci: replace mips with powerpc64, aarch64 and s390x

We drop our MIPS target because it no longer works.[1] We were
previously using it as a means of testing ripgrep in a big endian
environment. So to achieve that without MIPS, we test on powerpc64 and
s390x. (No particular reason to do both, but why not.)

We also add aarch64 as a proxy for at least ensuring everything works
for the same architecture as Apple silicon. It's not a guarantee that
everything works, but it seems better than nothing until we can actually
test Apple silicon in CI.

[1]: c788378d6f

ci/docker/s390x-unknown-linux-gnu/build

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+mkdir -p stage
+cp ../../ubuntu-install-packages ./stage/
+docker build -t burntsushi/cross:s390x-unknown-linux-gnu .