[rfa] Add cross platform i386 corefile support

[Andrew Cagney]

> This is a step into the direction that I was heading for.  However, I
> was envisioning a somewhat more generic approach, suitable for all
> i386 ELF targets.  I wasn't planning on having that support in 6.0
> though.  Would you mind if I keep your patch around until I implement
> that approach? 

BTW, there are somethings that can probably go in now:

- making the i386 REGNUMs into an enum

- x86-64 nat changes so that i386 on x86-64 works

thoughts on that?

Andrew