Setup an external toolchain with Yocto

You can use the sourcery g++ toolchain, but it requires custom tuning arguments to control selection of the multilib in the external toolchain sysroots, so you should switch to a separate layer and override the tuning files in question.

For example.

If you want to see an example of external toolchain stuff that works, see the meta-sourcery layer.

Watch this presentation about how to setup an external toolchain with oe-core.