base/regdump.h: Pass cooked register index to `REGDEF_GPX86_COMMON'.

I want to define `BP' and `SP' as macros, but they'll expand in the
calls to `REGDEF_GPX86_COMMON', which will break things.  They don't
expand directly in the body of `REGDEF_GPX86_XP' because `##' inhibits
this.  Alas, this means we have to do this token-gluing in the interface
macros rather than the common core.