bpo-31719: Fix test_regrtest.test_crashed() on s390x (#3912)

Add a new _testcapi._read_null() function to crash Python in a
reliable way on s390x.

On s390x, ctypes.string_at(0) returns an empty string rather than
crashing.
diff --git a/Lib/test/test_regrtest.py b/Lib/test/test_regrtest.py
index 264c74d..aae2743 100644
--- a/Lib/test/test_regrtest.py
+++ b/Lib/test/test_regrtest.py
@@ -543,6 +543,8 @@
                                 testname)
         self.assertEqual(output.splitlines(), all_methods)
 
+    @unittest.skipIf(sys.platform.startswith('aix'),
+                     "support._crash_python() doesn't work on AIX")
     def test_crashed(self):
         # Any code which causes a crash
         code = 'import test.support; test.support._crash_python()'