make the rwlock implementation MI.

each arch used to have to provide an rw_cas operation, but now we
have the rwlock code build its own version. on smp machines it uses
atomic_cas_ulong. on uniproc machines it avoids interlocked
instructions by using straight loads and stores. this is safe because
rwlocks are only used from process context and processes are currently
not preemptible in our kernel. so alpha/ppc/etc might get a benefit.

ok miod@ kettenis@ deraadt@

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