mips: Redefine LLSCSYNC as empty on non-Octeon MP.

This change deletes memory barriers on non-Octeon MP.  However, all
the appropriate acquire and release barriers are already used in
mutex stubs, and no barriers are needed in atomic_* unless we set
__HAVE_ATOMIC_AS_MEMBAR which we don't on MIPS.  So this should be
safe.

Unclear whether we need this even on Octeon -- don't have a clear
reference on why it's here.

(riastradh)