Drop s390x, glibc 2.12, _libgcc_mutex

[h-vetinari]

This picks up changes from #147, #162 & #164, and drops s390x on top, because we currently don't build packages for this target in conda-forge, and so building the compilers with every build is a bit overkill, especially since we haven't managed to get #147 green again despite several attempts.

Also drop old_triplet for reasons outlined here and _libgcc_mutex due to conda-forge/_libgcc_mutex-feedstock#6

Closes #162
Closes #164

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ The recipe is not parsable by parser conda-souschef (grayskull). This parser is not currently used by conda-forge, but may be in the future. We are collecting information to see which recipes are compatible with grayskull.
• ℹ️ The recipe is not parsable by parser conda-recipe-manager. The recipe can only be automatically migrated to the new v1 format if it is parseable by conda-recipe-manager.

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/13465346583. Examine the logs at this URL for more detail.}

[h-vetinari]

Reinstated the s390x recipe bits - PTAL.