[gn build] port 0da172b176 more

llvm/utils/gn/build/BUILD.gn

@@ -392,6 +392,12 @@ config("crt_code") {
     "-gline-tables-only",
     "-fvisibility=hidden",
   ]
+  assert(is_clang, "CRT code is always built with just-built clang")
+  cflags += [
+    "-Werror=thread-safety",
+    "-Werror=thread-safety-reference",
+    "-Werror=thread-safety-beta",
+  ]
 }
 
 config("warn_covered_switch_default") {