deps: V8: cherry-pick 70924d79f32c

Original commit message:

    [runtime] Fix non-conforming std::atomic_flag initialization

    std::atomic_flag has no constructor taking bool. Use ATOMIC_FLAG_INIT
    to ensure it works across different standard library implementations.

    Refs: #62572
    Change-Id: If5ff61339560aa0b9a2219aad9e5b6f2fd2b1155
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7874000
    Reviewed-by: Clemens Backes <clemensb@chromium.org>
    Commit-Queue: Joyee Cheung <joyee@igalia.com>
    Cr-Commit-Position: refs/heads/main@{#107728}

Refs: v8/v8@70924d7

Author: joyeecheung

common.gypi

@@ -40,7 +40,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.20',
+    'v8_embedder_string': '-node.21',
 
     ##### V8 defaults for Node.js #####
 

deps/v8/src/runtime/runtime-test.cc

@@ -1173,7 +1173,7 @@ RUNTIME_FUNCTION(Runtime_SetAllocationTimeout) {
   CONVERT_INT32_ARG_FUZZ_SAFE(timeout, 1);
   isolate->heap()->set_allocation_timeout(timeout);
 #else   // !V8_ENABLE_ALLOCATION_TIMEOUT
-  static std::atomic_flag printed_warning{false};
+  static std::atomic_flag printed_warning = ATOMIC_FLAG_INIT;
   if (!printed_warning.test_and_set()) {
     base::OS::PrintError(
         "Warning: %%SetAllocationTimeout has no effect in this build. Set the "