[libfuzzer] supporting libfuzzer on mac with asan

testing/libfuzzer/BUILD.gn

Index: testing/libfuzzer/BUILD.gn
diff --git a/testing/libfuzzer/BUILD.gn b/testing/libfuzzer/BUILD.gn
index 2063f9206e2ce49ca5a2688030f0a8191fbcb543..1ac14ecce3083cde5c0e56503938449d11590d05 100644
--- a/testing/libfuzzer/BUILD.gn
+++ b/testing/libfuzzer/BUILD.gn
@@ -37,6 +37,16 @@ static_library("libfuzzer_main") {
   }
 }
 
+# A config used by all fuzzer_tests.
+config("fuzzer_test_config") {
+  if (use_libfuzzer && is_mac) {
+    ldflags = [
+      "-Wl,-U,_LLVMFuzzerCustomMutator",
+      "-Wl,-U,_LLVMFuzzerInitialize",
+    ]
+  }
+}
+
 # Noop config used to tag fuzzer tests excluded from clusterfuzz.
 # Libfuzzer build bot uses this to filter out targets while
 # building an archive for clusterfuzz.