Make Bazel be able to reach ijar on Windows where it's called "ijar.exe" .

A small step towards the resolution of #276.

--
MOS_MIGRATED_REVID=114330517
diff --git a/tools/jdk/BUILD b/tools/jdk/BUILD
index 75edee6..50feeb0 100644
--- a/tools/jdk/BUILD
+++ b/tools/jdk/BUILD
@@ -35,6 +35,14 @@
     srcs = ["//external:javac"],
 )
 
+# On Windows, executables end in ".exe", but the label we reach it through
+# must be platform-independent. Thus, we create a little filegroup that
+# contains the appropriate platform-dependent file.
+filegroup(
+    name = "ijar",
+    srcs = glob(["ijar/*"]),
+)
+
 BOOTCLASS_JARS = [
     "rt.jar",
     "resources.jar",