ci: Add native PowerPC64LE and s390x jobs

We now have access to native runners, so make use of them for these
architectures. I kept the existing Docker job for ppc64le in case of
problems.

Author: tgross35

.github/workflows/main.yaml

@@ -70,8 +70,12 @@ jobs:
           os: ubuntu-24.04
         - target: powerpc64le-unknown-linux-gnu
           os: ubuntu-24.04
+        - target: powerpc64le-unknown-linux-gnu
+          os: ubuntu-24.04-ppc64le
         - target: riscv64gc-unknown-linux-gnu
           os: ubuntu-24.04
+        - target: s390x-unknown-linux-gnu
+          os: ubuntu-24.04-s390x
         - target: thumbv6m-none-eabi
           os: ubuntu-24.04
         - target: thumbv7em-none-eabi