(committed toplevel patch) Make rda native-only

[DJ Delorie]

> Or I could invoke patch reversion and demand that RDA *not* be built
> from the top level, at least in GCC's version of the top level.
> That also sounds entirely reasonable to me.

That sounds quite drastic to me.  If RDA doesn't build for you, take
it out of your local sources so it won't be built.  As for patch
reversion rules, the patch to add rda to gcc's configury did not break
anything in the gcc build, so there's no cause to revert it.