Add stdckdint.h header for C23

Add stdckdint.h header for C23

This patch adds <stdckdint.h> header, which defines ckd_{add,sub,mul}
using __builtin_{add,sub,mul}_overflow.  As requested, it doesn't
pedantically diagnose things which work just fine, e.g. inputs with
plain char, bool, bit-precise integer or enumerated types and
result pointer to plain char or bit-precise integer.
The header will #include_next <stdckdint.h> so that C library can supply
its part if the header implementation in the future needs to be split
between parts under the control of the compiler and parts under the
control of C library.

2023-08-12  Jakub Jelinek  <jakub@redhat.com>

	* Makefile.in (USER_H): Add stdckdint.h.
	* ginclude/stdckdint.h: New file.

	* gcc.dg/stdckdint-1.c: New test.
	* gcc.dg/stdckdint-2.c: New test.