Just to be clear, you're suggesting that if you have:
--build=powerpc-foo-bar --host=powerpc64-foo-bar --target=powerpc64-foo-bar
The user be able to specify something so that the build systems knows
the build machine can execute the host binaries, and a 4-stage
bootstrap should occur?
Well, that's possible, but I'd really prefer to avoid dealing with
"bootstraps" of cross-compilers at all and stick to native compilers, so
no, I'm not really suggesting that.
