ubsan: Don't -fsanitize=null instrument __seg_fs/gs pointers [PR111736]

On x86 and avr some address spaces allow 0 pointers (on avr actually
even generic as, but libsanitizer isn't ported to it and
I'm not convinced we should completely kill -fsanitize=null in that
case).
The following patch makes sure those aren't diagnosed for -fsanitize=null,
though they are still sanitized for -fsanitize=alignment.

2024-03-22  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/111736
	* ubsan.cc (ubsan_expand_null_ifn, instrument_mem_ref): Avoid
	SANITIZE_NULL instrumentation for non-generic address spaces
	for which targetm.addr_space.zero_address_valid (as) is true.

	* gcc.dg/ubsan/pr111736.c: New test.