Bazel: Support building with Java 9

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

BUILD

@@ -4,6 +4,17 @@ licenses(["notice"])
 
 exports_files(["LICENSE"])
 
+################################################################################
+# Java 9 configuration
+################################################################################
+
+config_setting(
+    name = "jdk9",
+    values = {
+        "java_toolchain": "@bazel_tools//tools/jdk:toolchain_jdk9",
+    },
+)
+
 ################################################################################
 # Protobuf Runtime Library
 ################################################################################
@@ -608,7 +619,10 @@ java_library(
     ]) + [
         ":gen_well_known_protos_java",
     ],
-    javacopts = ["-source 7", "-target 7"],
+    javacopts = select({
+       "//:jdk9": ["--add-modules=jdk.unsupported"],
+       "//conditions:default": ["-source 7", "-target 7"],
+    }),
     visibility = ["//visibility:public"],
 )