add bazel and cmake file for Half lib (#3906)

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 6698025..8530017 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -293,6 +293,13 @@
     path = "third_party/pffft",
 )
 
+maybe(
+    new_local_repository,
+    name = "half",
+    build_file = "build_tools/third_party/half/BUILD.overlay",
+    path = "third_party/half",
+)
+
 GOOGLE_RESEARCH_COMMIT = "a5213e2c92c3e87849fe417ba42786d0324e7c75"
 
 http_archive(