Don't call BarrierSetAssembler directly from MacroAssembler::access_* helpers

MacroAssembler::access_load_at() and MacroAssembler::access_store_at() are checking for raw access, and if so, call directly to corresponding BarrierSetAssembler methods. Instead they should do the proper virtual call, adorned with AS_RAW, and implementations should do the right thing accordingly, e.g. passing up the call without card-marking or such.