mips: Redefine BDSYNC as sync on Octeon, not syncw.

BDSYNC is used for membar_sync, which is supposed to be a full
sequential consistency barrier, which is not provided by syncw, so
this is necessary for correctness.

BDSYNC is not used for anything else, so this can't hurt performance,
except where it was necessary for correctness anyway or where the
semantic choice of membar_sync was too strong anyway.

(riastradh)