default triple to <arch>-linux-gnu

we'd need to be explicit during the bootstrap to use `<arch>-linux-musl`, but could set the cmake default triple to gnu.

https://blogs.gentoo.org/mgorny/2022/10/07/clang-in-gentoo-now-sets-default-runtimes-via-config-file/ also looks interesting but might need to distribute that in addition, which is a pain.