RFR: keep read barrier on final field access

[Roland Westrelin]

> I am probably confusing myself here. String.coder is treated specially along with other String
> fields via ciField::trust_static_non_final_fields. String.value is being explicitly casted in
> GraphKit to stable array to give the array contents stable properties?

    @Stable
    private final byte[] value;

    private final byte coder;

so String.value is stable. Not sure what the history is here. Is
UseImplicitStableValues useless now?

Roland.