[vm/concurrency] Use relaxed store when updating field list_length_offset.
[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 https://github.com/dart-lang/sdk/issues/47188

TEST=ci

Change-Id: I37e1aa4fc6ad8682210b3a76886a84673fb062cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214861
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk