Never use the "generic gnu" ponyc package

[SeanTAllen]

The "generic gnu" package is not actually generic. It is a glibc version that is built on "some glibc using Linux" and will work if it is installed on a Linux that is compatible at a library level with the system that we built the "generic gnu" package on.

Quite some time ago, Theo and I (who made the initial decision to include a "generic gnu") realized that the "generic gnu" idea was "a very bad idea".

There are a couple large problems with this approach:

1- anyone using ponyup on glibc distro that isn't compatible with our "generic gnu" platform will by default have things installed that will not work. That's a bad user experience.

2- in order to not fall horribly behind, from time to time, we need to "rev" the environment used to build "generic gnu" in. For us that has meant changing the Ubuntu version periodically. That then means that people who had an environment that worked will do an update and install a ponyc that doesn't work because it is for a different set of libraries. That's a bad user experience as well.

This PR drops "generic gnu" from being used by the ponyup installer and it avoids it being set as a target platform. Instead when the installer detects that it is being run on a glibc based Linux, it will attempt to identify the distro being used and if we support it, it will use the builds we do for that distro. In the case of "distros built on distros" like Linux Mint being built on Ubuntu, it will set the platform to the builds for the appropriate "base distro". If we are unable to determine the distro or the distro isn't supported, an error message is displayed that directs them to the Linux install instructions in the ponyc repo.

I will be doing a corresponding PR to ponyc to update the Linux section of INSTALL.md to account for the changes from this PR. Additionally, once this is merged, we can stop building the "generic gnu" package which I will also open a PR for.

[ponylang-main]

Hi @SeanTAllen,

The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 245.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.