libstdc++: Fix std::gcd and std::lcm for unsigned integers [PR 92978]

libstdc++: Fix std::gcd and std::lcm for unsigned integers [PR 92978]

This fixes a bug with mixed signed and unsigned types, where converting
a negative value to the unsigned result type alters the value. The
solution is to obtain the absolute values of the arguments immediately
and to perform the actual GCD or LCM algorithm on two arguments of the
same type.

In order to operate on the most negative number without overflow when
taking its absolute, use an unsigned type for the result of the abs
operation. For example, -INT_MIN will overflow, but -(unsigned)INT_MIN
is (unsigned)INT_MAX+1U which is the correct value.

libstdc++-v3/ChangeLog:

	PR libstdc++/92978
	* include/std/numeric (__abs_integral): Replace with ...
	(__detail::__absu): New function template that returns an
	unsigned type, guaranteeing it can represent the most
	negative signed value.
	(__detail::__gcd, __detail::__lcm): Require arguments to
	be unsigned and therefore already non-negative.
	(gcd, lcm): Convert arguments to absolute value as unsigned
	type before calling __detail::__gcd or __detail::__lcm.
	* include/experimental/numeric (gcd, lcm): Likewise.
	* testsuite/26_numerics/gcd/gcd_neg.cc: Adjust expected
	errors.
	* testsuite/26_numerics/lcm/lcm_neg.cc: Likewise.
	* testsuite/26_numerics/gcd/92978.cc: New test.
	* testsuite/26_numerics/lcm/92978.cc: New test.
	* testsuite/experimental/numeric/92978.cc: New test.