Refactor osGroup / osIdentifier pairs to osGroup / osSubgroup (dotnet/coreclr#26583)

Refactor osGroup / osIdentifier pairs to osGroup / osSubgroup (dotnet/coreclr#26583)

Refactor osGroup / osIdentifier pairs to osGroup / osSubgroup

This simple cleanup removes the duplication between osGroup and
osIdentifier by switching over to use the pair osGroup and osSubgroup
such that the final OS identifier is a plain concatenation of osGroup
and osSubgroup. As the only currently supported OS flavors using
OS subgroups are Linux_musl and Linux_rhel6, this simplifies the
platform-matrix.yml and related scripts overall.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/1a17ea321bb9489d85eb7731d765ae36529cab6d