See the comments for addupc_intr() and the NetBSD implementation.
We use dummy versions of fuswintr() and susiwintr(), so addupc_intr()
always pushes the work to trap() (this is inefficient), and trap()
calls the special i386 function addupc() instead of addupc_task().
addupc() is more efficient than addupc_intr(), so some of the lost
efficiency is recovered. However, addupc() may be broken on plain
i386's since it doesn't check for write permission like copyout().
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