x86 update -- raytrace benchmark

[Matthew Fluet]

> Now for the bad news.  I tried to compile the native version with
> -DMLton_safe=0 and the resulting executable terminates immediately on
> every example with exit status zero without producing any output. 
> Matthew, any ideas? 

I started looking into this over the weekend.  I've figured out that it's
something funny going on with IO.  Using -DMLton_safe=0 is also failing on
something as simple as:
val _ = print "Hello World!\n"
Although failing isn't quite right; there is a call to Posix_IO_write, but
it seems that pointer being passed isn't pointing to the right place.  I
suspect that something similar is going on in the other direction --
the results of some read are not being copied to the right place and I
suspect that it looks like you've reached the end-of-file right at the
first read.