Building crosscompilers was a pain in the ass. Up to gentoo's crossdev.sh script. Here's a patch for crossdev.sh for StrongARM CPUs - it should do for other ARMs either, just change the -mcpu and -mtune flags. It patches a tiny bug in glibc-2.3.2 and adds ARM support into crossdev.sh.
Actually the patch does one more thing: the original script copies (!) all source trees from /var/tmp/portage/[binutils|gcc|glibc|linux-headers] to /var/tmp/portage/crossdevbuild. The cp ist replaced by a mv (probably not the gentoo way, but saves me space on the HDD during build time).