Add powerpc64-unknown-freebsd target

Per rust-lang/stdarch#765 add powerpc64-unknown-freebsd to prevent further breakages.

Author: pkubaj

ci/build.sh

@@ -81,6 +81,7 @@ mipsel-unknown-linux-gnu \
 mipsel-unknown-linux-gnu \
 mipsel-unknown-linux-musl \
 powerpc-unknown-linux-gnu \
+powerpc64-unknown-freebsd \
 powerpc64-unknown-linux-gnu \
 powerpc64le-unknown-linux-gnu \
 s390x-unknown-linux-gnu \