Aside from mutex, BSD always implements sx locks. They are a form of rwlock,
and according to BSD's sx.h, the differences from their own rwlock are an
implementation detail. Let's just use them as rwlocks for now.

The declarations are uglier than I wanted. But this file ends up being included
from c and cc code, and rwlock.h calls condvar.h inside - which is full of
sync_stub.h - and as a result condvar.h itself - are usually included in extern
"C" blocks. Because we are not expected to use sxlocks from C, it should be
fine.