libstdc++: Update __cpp_lib_three_way_comparison macro

With P1614R2 fully implemented (except for the <chrono> types which we
don't support at all) we can define the feature test macro to the new
value.

	* include/std/version (__cpp_lib_three_way_comparison): Update value.
	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.