pmb.aportgen.gcc: add subpkg libstdc++-dev-$arch
Add the subpackage, because g++-$arch depends on it since
12.1.1_git20220630-r6. This fix was already applied to pmaports, thanks
to ungeskriptet! Adjust the aportgen code to make sure this is added in
the future as well.
- Related: pmaports!3464
- Related: https://gitlab.alpinelinux.org/alpine/aports/-/commit/f1044b1dae5a8bde2b1e032a609dfe0ba86aa490