[vm/concurrency] Use relaxed atomics when accessing usage counters - for TSAN

For now our decision is to not synchronize access to usage counters and
instead let there be possible non-observable writes (if two concurrent writes
happen).

Though TSAN will report concurrent access in C++ to the usage counters
as a race, so we'll use a relaxed atomic.

(We do have non-synchronized access to the usage counters already:
If the mutator kicks of a background compilation, the BG compiler
might reset the counters while mutator is running.)

Closes https://github.com/dart-lang/sdk/issues/45134

TEST=Fixes flaky tsan errors.

Change-Id: I7919747536d43094e7201e03f464e4ac7b1a4740
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/187923
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>