message: Update OPENSSL_CC_powerpc64-unknown-linux-gnu
  With the new `rust-slave-linux-cross:2017-02-02`, powerpc64 no longer
  uses gcc-5.  I updated the `Dockerfile` at the time to reflect this, but
  missed `OPENSSL_CC_powerpc64-unknown-linux-gnu` in `Makefile.in`.
diff: @@ -251,7 +251,7 @@ OPENSSL_CC_mips64-unknown-linux-gnuabi64 := mips64-linux-gnuabi64-gcc
  OPENSSL_CC_mips64el-unknown-linux-gnuabi64 := mips64el-linux-gnuabi64-gcc
  OPENSSL_CC_mipsel-unknown-linux-gnu := mipsel-linux-gnu-gcc
  OPENSSL_CC_powerpc-unknown-linux-gnu := powerpc-linux-gnu-gcc
  -OPENSSL_CC_powerpc64-unknown-linux-gnu := powerpc64-linux-gnu-gcc-5
  +OPENSSL_CC_powerpc64-unknown-linux-gnu := powerpc64-linux-gnu-gcc
  OPENSSL_CC_powerpc64le-unknown-linux-gnu := powerpc64le-linux-gnu-gcc
  OPENSSL_CC_s390x-unknown-linux-gnu := s390x-linux-gnu-gcc
  OPENSSL_CC_x86_64-apple-darwin := clang
  