gh-133157: remove usage of `_Py_NO_SANITIZE_UNDEFINED` in `faulthandler` (#134047)

In `faulthandler_sigfpe()`, instead of using 1/0 arithmetic, we explicitly raise SIGFPE.
We also remove `faulthandler._read_null()` since reading from NULL is an undefined
behavior and `faulthandler` should not check for low-level C undefined behaviors.