Build rpcgen-generated files when cross compiling

[Roland McGrath]

I'd like to see the version that always builds an rpcgen for the build
host.  If that version of the change is suficiently clean and
straightforward, I think I'll prefer it.  It means less difference between
cross and native cases.  It also further distinguishes building libc from
testing it, which is just nice separation of concerns, and also gets closer
to "almost-native" cases being "native enough" cases (e.g. building in a
chroot but with a host kernel not as new as the --enable-kernel setting).


Thanks,
Roland