[rfa/arm] Eliminate REGISTER_CONVERTIBLE() from ARM

[Andrew Cagney]

> Andrew, I will have to apply your patch and run some tests.  And we need
> to hear from the Linux/ARM folks as well.
> 
> Or have you run the testsuite against a, lets say, arm-elf target
> already
> (ARMulator is enough)?
> 
> I'll get there.  However, I suspect the results will be pretty meanginless.  When I last touched that code (to ``fix'' the register FP types to be arm_ext) I found the results went sideways - a host{be,le} VS targ{be,le} table showed improvements in some cases but no change in others :-/

Yep, no real change.  The new version had three (bogus) regressions in 
completion.exp because the directory path was different :-(

I've checked it in,
Andrew