[vm/concurrency] Use relaxed store when updating field list_length_offset.

This is to satisfy TSAN where it sees a data race between updating a field on RecordStore and loading the field for cloning. We already use relaxed stores for Field's guarded_cid updates.

Fixes #47188

TEST=ci

Change-Id: I37e1aa4fc6ad8682210b3a76886a84673fb062cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214861
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/vm/object.cc

@@ -10867,8 +10867,9 @@ intptr_t Field::guarded_list_length_in_object_offset() const {
 void Field::set_guarded_list_length_in_object_offset_unsafe(
     intptr_t list_length_offset) const {
   ASSERT(IsOriginal());
-  StoreNonPointer(&untag()->guarded_list_length_in_object_offset_,
-                  static_cast<int8_t>(list_length_offset - kHeapObjectTag));
+  StoreNonPointer<int8_t, int8_t, std::memory_order_relaxed>(
+      &untag()->guarded_list_length_in_object_offset_,
+      static_cast<int8_t>(list_length_offset - kHeapObjectTag));
   ASSERT(guarded_list_length_in_object_offset() == list_length_offset);
 }