Adding libyaml submodule and getting it building. (#6319)

The yaml_util.* files are here to ensure the cmake/bazel configuration is
correct and don't yet do anything meaningful.
diff --git a/build_tools/bazel/workspace.bzl b/build_tools/bazel/workspace.bzl
index 076c3a3..49ed2d6 100644
--- a/build_tools/bazel/workspace.bzl
+++ b/build_tools/bazel/workspace.bzl
@@ -35,6 +35,13 @@
         path = paths.join(iree_path, "third_party/flatcc"),
     )
 
+    maybe(
+        native.new_local_repository,
+        name = "com_github_yaml_libyaml",
+        build_file = iree_repo_alias + "//:build_tools/third_party/libyaml/BUILD.overlay",
+        path = paths.join(iree_path, "third_party/libyaml"),
+    )
+
     # TODO(scotttodd): TensorFlow is squatting on the vulkan_headers repo name, so
     # we use a temporary one until resolved. Theirs is set to an outdated version.
     maybe(