It is my understanding that the mesa radeonsi driver is separate from r600 and it's called literally "radeonsi".
So for mesa you would use something like that for intel + radeonsi. swrast is good to have there because the build system seems to sometimes depend on it. If you don't want the intel driver, you can still keep --with-dri-drivers= and just let a space after because otherwise it will use some defaults.
And xf86-video-ati needs to be built with glamor, so install glamor first and then compile xf86-video-ati with (and perhaps use --disable-exa, I think it will not be used at all with radeonsi). Glamor does not work with X.org 1.13 and later yet as far as I know.
./autogen.sh --prefix=/usr \
I think this are the only things you need to modify besides having the latest libdrm and xf86-video-ati and glamor git versions.