Fixes: #4256.

Bazel@HEAD supports Java 9.

The current code has one single issue with Java 9 compliance: the usage
of sun.misc package. We add jdk.unsupported module with --add-modules
compiler option for now. Long term, the usage of non public API should
be avoided.

To build with Java 9, build custom bazel version and issue:

  $ bazel --host_javabase=/usr/lib64/jvm/java-9-openjdk build \
    --javacopt='--release 9' \
    --java_toolchain=@bazel_tools//tools/jdk:toolchain_jdk9 \
   :protobuf_java