I broke `pkgsMusl` with #209870.
Something odd is happening with `xgcc` (the temporary compiler that
should be used only to compile `gcc`, although we are using it to
compile a temporary `patchelf` too) and `libstdc++`.
The temporary fix in this commit is to use `-static-libstdc++` for
the ephemeral `patchelf` built by `xgcc`. It isn't pretty, but it
appears to work.
Incorporates:
- https://github.com/NixOS/nixpkgs/pull/224945
3778e07994