loong64: build: Fix libc link flags

src/build/config/BUILD.gn

@@ -216,11 +216,14 @@ config("default_libs") {
     # linking can have run-time side effects, nothing should be listed here.
     libs = []
   } else if (is_linux || is_chromeos) {
-    libs = [
-      "dl",
-      "pthread",
-      "rt",
-    ]
+    # loong64 uses newer libc that subsumes libdl.so etc.
+    if (current_cpu != "loong64") {
+      libs = [
+        "dl",
+        "pthread",
+        "rt",
+      ]
+    }
   }
 }