bpo-39882: Py_FatalError() logs the function name (GH-18819)

The Py_FatalError() function is replaced with a macro which logs
automatically the name of the current function, unless the
Py_LIMITED_API macro is defined.

Changes:

* Add _Py_FatalErrorFunc() function.
* Remove the function name from the message of Py_FatalError() calls
  which included the function name.
* Update tests.