x86 codegen bug

[Matthew Fluet]

> The call to GC_gc in x86-mlton.fun needs to pass along a filename and line
> argument.

That's not going to be easy to incorporate.  File name alone isn't too
bad; I changed the result of the outputC and outputS functions to also
return the file name, which I can add add as a .string pseudoop at the top
of each assembly file.  Line numbers are a little harder.  The simplest
hack would be to produce .S files which will go through the preprocessor,
and push the label __LINE__ when calling GC_gc.  I guess I could also do
that with __FILE__ to get the file name.