sparc64: Fix membar_sync by issuing membar #StoreLoad.

In TSO this is the only memory barrier ever needed, and somehow we
got this wrong and instead issued an unnecessary membar #LoadLoad --
not needed even in PSO let alone in TSO.

XXX Apparently we may run userland programs with PSO or RMO, in which
case all of these membars need fixing:

                        PSO                    RMO
membar_consumer        nop                    membar #LoadLoad
membar_producer        membar #StoreStore      membar #StoreStore
membar_enter            nop                    membar #LoadLoad|LoadStore
membar_exit            membar #StoreStore      membar #LoadStore|StoreStore
membar_sync            membar #StoreLoad|StoreStore
                                                membar #...everything...

But at least this fixes the TSO case in which we run the kernel.
Also I'm not sure there's any non-TSO hardware out there in practice.

(riastradh)