patch applied to sh.md: add prefetch pattern

[DJ Delorie]

> > One of my internal builds is failing prefetch tests because this "new"
> > pattern faults on an invalid address.  The GCC docs say that this
> > pattern must not fault on invalid addresses.  Is the pattern wrong, or
> > is the simulator wrong?
> 
> The simulator is wrong.

Ok, thanks.  Question: does the SH *chip* have faulting and
nonfaulting opcodes, or just one type?  I'm thinking, if the chip
doesn't fault, then having the simulator fault might be considered a
debugging aid rather than a bug.