How do I build i386 arch on Athlon 64?

[Gary W. Swearingen]

I finally found a clue right there in /usr/src/Makefile, where
it talks about "TARGET_ARCH".  But when I used it with i386, it seemed
to be compiling for the default gcc (386, I suppose), even when I
had CPUTYPE=athlon-xp in "/etc/make.conf".  This seems to be doing
the job (though it's still building):

make TARGET_ARCH=i386 TARGET_CPUTYPE=athlon-xp <make-target>