bpo-36974: Fix GDB integration (GH-13665)

As it changes the way functions are called, the PEP 590 implementation
skipped the functions that the GDB integration is looking for
(by name) to find function calls.

Looking for the new helper `cfunction_call_varargs` hopefully fixes the
tests, and thus buildbots.

The changed frame nuber in test_gdb is due to there being fewer
C calls when calling a built-in method.

Tools/gdb/libpython.py

@@ -1564,7 +1564,8 @@ class Frame(object):
             return False
 
         if caller in ('_PyCFunction_FastCallDict',
-                      '_PyCFunction_FastCallKeywords'):
+                      '_PyCFunction_FastCallKeywords',
+                      'cfunction_call_varargs'):
             arg_name = 'func'
             # Within that frame:
             #   "func" is the local containing the PyObject* of the