Add _PyThreadState_UncheckedGet()

Issue #26154: Add a new private _PyThreadState_UncheckedGet() function which
gets the current thread state, but don't call Py_FatalError() if it is NULL.

Python 3.5.1 removed the _PyThreadState_Current symbol from the Python C API to
no more expose complex and private atomic types. Atomic types depends on the
compiler or can even depend on compiler options. The new function
_PyThreadState_UncheckedGet() allows to get the variable value without having
to care of the exact implementation of atomic types.

Changes:

* Replace direct usage of the _PyThreadState_Current variable with a call to
  _PyThreadState_UncheckedGet().
* In pystate.c, replace direct usage of the _PyThreadState_Current variable
  with the PyThreadState_GET() macro for readability.
* Document also PyThreadState_Get() in pystate.h

Modules/faulthandler.c

@@ -490,7 +490,7 @@ faulthandler_thread(void *unused)
         assert(st == PY_LOCK_FAILURE);
 
         /* get the thread holding the GIL, NULL if no thread hold the GIL */
-        current = (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
+        current = _PyThreadState_UncheckedGet();
 
         _Py_write_noraise(thread.fd, thread.header, (int)thread.header_len);