libstdc++: Rewrite std::variant comparisons without macros

libstdc++: Rewrite std::variant comparisons without macros

libstdc++-v3/ChangeLog:

	* include/std/variant (__detail::__variant::__compare): New
	function template.
	(operator==, operator!=, operator<, operator>, operator<=)
	(operator>=): Replace macro definition with handwritten function
	calling __detail::__variant::__compare.
	(operator<=>): Call __detail::__variant::__compare.