Move j2objc helper scripts into open-source Bazel.

--
MOS_MIGRATED_REVID=100490916

tools/objc/BUILD

diff --git a/tools/objc/BUILD b/tools/objc/BUILD
index 2033ce7..7f87137 100644
--- a/tools/objc/BUILD
+++ b/tools/objc/BUILD
@@ -79,9 +79,7 @@
 )
 
 # TODO(bazel-team): Open-source the script once J2ObjC support is open-sourced.
-genrule(
+py_library(
     name = "j2objc_dead_code_pruner",
-    srcs = [],
-    outs = ["dummy_dead_code_pruner.py"],
-    cmd = "touch $(OUTS)",
+    srcs = ["j2objc_dead_code_pruner.py"],
 )