Implement a workaround for thread_local on aarch64

Some versions of lld break thread_local on aarch64. There's a
minimally-painful workaround for it, so let's apply that.

Change-Id: I706de9e280cc15006e5767ee10d961cb2d99352c
diff --git a/aos/BUILD b/aos/BUILD
index a46911e..04c0548 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -535,3 +535,11 @@
         "@com_google_absl//absl/strings:str_format",
     ],
 )
+
+cc_library(
+    name = "thread_local",
+    hdrs = [
+        "thread_local.h",
+    ],
+    visibility = ["//visibility:public"],
+)