gh-81057: Add a CI Check for New Unsupported C Global Variables (gh-102506)

This will keep us from adding new unsupported (i.e. non-const) C global variables, which would break interpreter isolation.

FYI, historically it is very uncommon for new global variables to get added. Furthermore, it is rare for new code to break the c-analyzer. So the check should almost always pass unnoticed.

Note that I've removed test_check_c_globals. A test wasn't a great fit conceptually and was super slow on debug builds. A CI check is a better fit.

This also resolves gh-100237.

https://github.com/python/cpython/issues/81057