libstdc++: Define C++26 saturation arithmetic functions (P0543R3)

libstdc++: Define C++26 saturation arithmetic functions (P0543R3)

This was approved for C++26 last week at the WG21 meeting in Kona.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def (saturation_arithmetic): Define.
	* include/bits/version.h: Regenerate.
	* include/std/numeric: Include new header.
	* include/bits/sat_arith.h: New file.
	* testsuite/26_numerics/saturation/add.cc: New test.
	* testsuite/26_numerics/saturation/cast.cc: New test.
	* testsuite/26_numerics/saturation/div.cc: New test.
	* testsuite/26_numerics/saturation/mul.cc: New test.
	* testsuite/26_numerics/saturation/sub.cc: New test.
	* testsuite/26_numerics/saturation/version.cc: New test.