If you google around, there are some guides on building gcc for RX, but none of them is really concise. So here you go.
With this method I was able to compile a toolchain for RX on Mac OS 10.8.2.
This post for Linux on the Renesas forums basically includes everything mentioned here, but with a lot of overhead and not very well formatted, however head there if you want some more explanations on the options used. Also thanks to Andy K. who helped me troubleshooting.