bpo-37499: Test various C calling conventions (GH-15776)

Add functions with various calling conventions to `_testcapi`, expose them as module-level functions, bound methods, class methods, and static methods, and test calling them and introspecting them through GDB.


https://bugs.python.org/issue37499


Co-authored-by: Jeroen Demeyer <[email protected]>
Automerge-Triggered-By: @pganssle

Author: 2 people