Is there a general guideline for cross-building on osx?

I'm trying to build i386 -current but if I say -m i386 tools, it will
fail in "gas".  If I do tools with -m macppc, they build, but then a
-m i386 distribution will fail.  I feel like I'm missing something
easy, but the tools differences is confusing because I thought
build.sh would magically know which tools it needed to build.

thanks,
_Matt