stagebase: set basearch to gentoo arch

[dwfreed]

I'll rewrite this some other time, but this fixes an issue with distkernel support (dracut gets called with "--kernel-image=/usr/src/linux-6.6.32-gentoo-dist/unsupported ARCH="). Having the base Gentoo arch available is useful either way.

[akhuettel]

This makes sense to me but I don't fully understand the impact yet.