Add entry point support to as86 backend
I needed entry point support with the as86 format, and after looking through
the archives found a similar desire from someone in 2002. For some reason
such a patch never made it into the code, even though the required flag
value is present, so I offer the a patch of my own.
I compared against what is done in the .obj format and the approaches are
quite similar which I hope will aid in its acceptability. While I have
tested it extensively it does do the job asked, and I'm honestly not sure
what extensive testing of the change would look like.