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/CMakeLists.txt b/CMakeLists.txt
index b4bb2d5..ca49b92 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -436,8 +436,9 @@
 
 add_subdirectory(build_tools/third_party/cuda EXCLUDE_FROM_ALL)
 add_subdirectory(build_tools/third_party/flatcc EXCLUDE_FROM_ALL)
-add_subdirectory(build_tools/third_party/vulkan_memory_allocator EXCLUDE_FROM_ALL)
+add_subdirectory(build_tools/third_party/libyaml EXCLUDE_FROM_ALL)
 add_subdirectory(build_tools/third_party/stblib EXCLUDE_FROM_ALL)
+add_subdirectory(build_tools/third_party/vulkan_memory_allocator EXCLUDE_FROM_ALL)
 
 add_subdirectory(third_party/googletest EXCLUDE_FROM_ALL)
 if(${IREE_ENABLE_THREADING})