cppgc: fix compilation error on older gcc

Fixes the following compilation error:
'atomic_uint8_t' in namespace 'std' does not name a type

Change-Id: Ie231b544cafc694271f50f4841b823e8cff3ec1d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2434390
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Reviewed-by: Junliang Yan <junyan@redhat.com>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70165}

src/heap/cppgc/default-job.h

@@ -101,7 +101,7 @@ class DefaultJobImpl {
   std::unique_ptr<cppgc::JobTask> job_task_;
   std::vector<std::shared_ptr<Thread>> job_threads_;
   std::atomic_bool can_run_{true};
-  std::atomic_uint8_t active_threads_{0};
+  std::atomic<uint8_t> active_threads_{0};
 
   // Task id management.
   v8::base::Mutex ids_lock_;