[vm/slot] Use seq_cst memory order for failure-branch of compare_exchange.
[vm/slot] Use seq_cst memory order for failure-branch of compare_exchange.

This fixes flutter engine test failure on Windows where it complained about invalid load memory order for failure branch(default=acq_rel) for this operation.
acq_rel is not allowed for failed comparison test memory order(per https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange)

TEST=flutter/engine runtime_unittests on Windows

Change-Id: I59dd3aed04f483b2e2a82721e337b78b8e3190d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175101
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Reviewed-by: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@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