Allow mixing libc and raw backend on Linux (at least for linux_execfn)

At uutils/coreutils, AT_EXECFN is used at multi-call binary to match with AppArmor's policy (avoid argv[0] hijack) and raw backend is used to avoid `LD_PRELOAD` injection. However, `LD_PRELOAD` is required by some GnuTests. It can be avoided if `rustix::raw::param::linux_execfn()` is possible.