sh: Optimize runtime disabling of trapped I/O.

Presently trapped I/O is only registered if it's not explicitly disabled
for the platforms that select it openly. From the fault path this runs
through an address lookup before figuring out that nothing matches and
falls back through the error path, but we can forego the lookup
completely by testing if it's been explicitly disabled. This provides a
measurable speedup for things like qemu that rely on runtime disabling.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>