Segment register support for the i386

[Joern Rennecke]

> > I believe my original thought was to do this with extra modes, so that
> > you'd have P_Xmode and P_Ymode (P_GSmode and P_FSmode in the x86 case)
> > to disambiguate the different address spaces.
> 
> It's a good solution for harvard architectures too.
> (DSP's and micro controllers)
> Something like PCODEmode and PDATAmode.

We have some precedent for target-specific modes with EXTRA_CC_MODES
and SELECT_CC_MODE.  However, Pmode is used quite a lot, not only in
the rtl generation stage, but also in some optimizers.  So we'd need
a macro to detect if a mode belongs to the Pmode 'family', and teach
the optimizers how to use it.

How about POINTER_SIZE / POINTERS_EXTEND_UNSIGNED ?  Do we want to
keep them applying to all pointer modes?