(committed toplevel patch) Make rda native-only

[Nathanael Nerode]

Alex Oliva said:
>This is not right.  rda is supposed to work in a cross set up.  If it
>doesn't, there's something wrong for the particular target you're
>building, which you failed to mention.  That would be a bug in rda, or
>in the top level for failing to exclude rda from a broken set up.

i386-cygwin
powerpc-eabi
powerpc-linux

Need I go on?  RDA crashes cross configurations very consistently for 
me.  I'm sure it's a bug in RDA, but it's damaging my attempts to work 
on gcc, gdb, binutils, and anything else, and I'm not about to work on 
RDA.

I'll be happy to exclude RDA from building on every system I've ever 
tried to build a cross to, but that seemed more drastic.

--Nathanael