Fix atomic initialization to work with C++20.

Bug: chromium:1284275
Change-Id: I727950c0bed499050ff550e90b9da07404057abe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3630348
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80454}

src/trap-handler/handler-shared.cc

@@ -36,7 +36,12 @@ size_t gNumCodeObjects = 0;
 CodeProtectionInfoListEntry* gCodeObjects = nullptr;
 std::atomic_size_t gRecoveredTrapCount = {0};
 
+#if !defined(__cpp_lib_atomic_value_initialization) || \
+    __cpp_lib_atomic_value_initialization < 201911L
 std::atomic_flag MetadataLock::spinlock_ = ATOMIC_FLAG_INIT;
+#else
+std::atomic_flag MetadataLock::spinlock_;
+#endif
 
 MetadataLock::MetadataLock() {
   if (g_thread_in_wasm_code) {

src/wasm/module-compiler.cc

@@ -120,16 +120,22 @@ class CompilationUnitQueues {
     // Add one first queue, to add units to.
     queues_.emplace_back(std::make_unique<QueueImpl>(0));
 
+#if !defined(__cpp_lib_atomic_value_initialization) || \
+    __cpp_lib_atomic_value_initialization < 201911L
     for (auto& atomic_counter : num_units_) {
       std::atomic_init(&atomic_counter, size_t{0});
     }
+#endif
 
     top_tier_compiled_ =
         std::make_unique<std::atomic<bool>[]>(num_declared_functions);
 
+#if !defined(__cpp_lib_atomic_value_initialization) || \
+    __cpp_lib_atomic_value_initialization < 201911L
     for (int i = 0; i < num_declared_functions; i++) {
       std::atomic_init(&top_tier_compiled_.get()[i], false);
     }
+#endif
   }
 
   Queue* GetQueueForTask(int task_id) {
@@ -298,7 +304,10 @@ class CompilationUnitQueues {
 
   struct BigUnitsQueue {
     BigUnitsQueue() {
+#if !defined(__cpp_lib_atomic_value_initialization) || \
+    __cpp_lib_atomic_value_initialization < 201911L
       for (auto& atomic : has_units) std::atomic_init(&atomic, false);
+#endif
     }
 
     base::Mutex mutex;