Use powerpc-unknown-linux-gnu to test big-endian

The mips64 target we used before has been demoted to tier 3:

rust-lang/rust#115218

Author: djc

.github/workflows/rust.yml

@@ -36,7 +36,7 @@ jobs:
         target:
           - aarch64-unknown-linux-gnu
           - i686-unknown-linux-gnu
-          - mips64-unknown-linux-gnuabi64
+          - powerpc-unknown-linux-gnu
     steps:
       - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1