GN: Add option to build with TSAN

runtime/BUILD.gn

Index: runtime/BUILD.gn
diff --git a/runtime/BUILD.gn b/runtime/BUILD.gn
index 6008aecabad0430312d83ca6f0f48e5cbc0a6c30..e7c13da6f7218a4324a7454c52a1b92c6eeb66c6 100644
--- a/runtime/BUILD.gn
+++ b/runtime/BUILD.gn
@@ -168,8 +168,15 @@ config("dart_config") {
       cflags += [ "-O3" ]
     }
 
+    ldflags = []
     if (defined(is_asan) && is_asan) {
-      ldflags = [ "-fsanitize=address" ]
+      ldflags += [ "-fsanitize=address" ]
+    }
+    if (defined(is_msan) && is_msan) {
+      ldflags += [ "-fsanitize=memory" ]
+    }
+    if (defined(is_tsan) && is_tsan) {
+      ldflags += [ "-fsanitize=thread" ]
     }
   }
 }