Make output of sysconfig.get_platform() more generic

"sysconfig.get_platform()" returns "os390-29.00-3931", which is problematic for distributing Python packages (.whl) as it means if a system is a on slightly different machine or older z/OS version it will be unable to install on that system and will only install on the build system if it's a python package that is not "pure" (which means it contains something like C code). If you build a python package with the os390 tag it will complain, because it compares it with sysconfig.get_platform():
ERROR: pysear-0.3.0-cp313-cp313-os390.whl is not a supported wheel on this platform.

If you run sysconfig.get_platform() on Linux it will return something like "linux-x86_64" which is more generic and will allow for a more portable build. Windows also gives a more generic response:
"win-amd64"

I would consider this a bug since every other platform does this differently and since this requires you to do these workarounds that are cumbersome and not best practice:
- Setting root to pure (this is not a good idea since it's effectively lying about what the package contains, root_is_pure indicates that there is no C or Rust code in the package)
- Renaming the package to achieve the same thing as setting root_is_pure to true (this is again also lying and not best practice)

I ran into this issue because I built a package on a system that returned os390-29.00-3932 and it wouldn't work on a system that returned os390-29.00-3931 (despite being same hardware generation, both z16 machines from what I understood).

Other information:
- Both build and install system were on z/OS 3.1 and were z16 machines
- This is an issue in both Python 3.12 and Python 3.13

This causes issues with PyPi.org because it doesn't understand tags like os390-29.00-3932 and os390-29.00-3931, so you can't release z/OS open source software on PyPi if it contains C code or other compiled code, or distribute it to a machine with a slightly different hardware revision. I think sysconfig.get_platform() should be changed so it returns something more generic like "os390_zarch", "os390_s390", or "os390".