[rx] add register-specific constraints

[DJ Delorie]

> Source-level asms can use registers.  E.g.
> 
>   register int r1 __asm__("r1");
>   asm ("" : "=r"(r1));
> 
> That's guaranteed to not replace the hard register in the asm.
> Glibc and the linux kernel relies on this for a number of ports,
> so it's unlikely to ever break.
> 
> Is there any other reason to add these?

Does that work for register pairs?