libstdc++: Fix deleted overload of __absu(bool)

libstdc++: Fix deleted overload of __absu(bool)

libstdc++-v3/ChangeLog:

	* include/std/numeric (__detail::__absu(bool)): Make deleted
	function a function template, so it will be chosen for calls
	with an explicit template argument list.
	* testsuite/26_numerics/gcd/gcd_neg.cc: Add dg-prune-output.
	* testsuite/26_numerics/lcm/lcm_neg.cc: Likewise.