fix x86 darwin

[Mike Stump]

This fixes the builds on x86 darwin.  They've been broken since:

2004-06-08  Zdenek Dvorak  <rakdver@atrey.karlin.mff.cuni.cz>

         PR rtl-optimization/15717
         * config/i386/i386.c (legitimate_constant_p): Do not allow
         x - symbol_ref.

and getting worse.  :-(

I developed my own version, and then I found Andrew had a version, 
while investigating, I found Stuart had a patch to fix a recent 
breakage introduced by Mark in the great darwin.c rewrite.  I folded 
all the aspects of them together and ended up with the below...

Ok?

Bootstrapped on i686-apple-darwin8.0.0b1.

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