ci: drop mips, add powerpc and s390x

The main reason we used mips before was to get test coverage on a big
endian target. Now that mips no longer seems to work[1], I wanted to
add at least one other big endian target. From the tier 2 supported
platforms[2], the only big endian targets I could find were powerpc and
s390x. So we just add both here.

[1]: rust-lang/compiler-team#648
[2]: https://doc.rust-lang.org/nightly/rustc/platform-support.html#tier-2-with-host-tools

.github/workflows/ci.yml

@@ -54,10 +54,14 @@ jobs:
           os: ubuntu-latest
           rust: stable
           target: i686-unknown-linux-gnu
-        - build: stable-mips
+        - build: stable-powerpc64
           os: ubuntu-latest
           rust: stable
-          target: mips64-unknown-linux-gnuabi64
+          target: powerpc64-unknown-linux-gnu
+        - build: stable-s390x
+          os: ubuntu-latest
+          rust: stable
+          target: s390x-unknown-linux-gnu
         - build: beta
           os: ubuntu-latest
           rust: beta