libstdc++: Define operator<=> for <system_error> types

libstdc++: Define operator<=> for <system_error> types

Another piece of P1614R2 for C++20.

This also adds tests for operator< in C++11, which was present but
untested.

	* include/std/system_error (error_category::operator<=>)
	(operator<=>(const error_code&, const error_code&))
	(operator<=>(const error_condition&, const error_condition&)): Define
	for C++20.
	* testsuite/19_diagnostics/error_category/operators/less.cc: New test.
	* testsuite/19_diagnostics/error_category/operators/three_way.cc: New
	test.
	* testsuite/19_diagnostics/error_code/operators/equal.cc: Remove
	incorrect comment.
	* testsuite/19_diagnostics/error_code/operators/less.cc: New test.
	* testsuite/19_diagnostics/error_code/operators/not_equal.cc: Remove
	incorrect comment.
	* testsuite/19_diagnostics/error_code/operators/three_way.cc: New test.
	* testsuite/19_diagnostics/error_condition/operators/equal.cc: Remove
	incorrect comment.
	* testsuite/19_diagnostics/error_condition/operators/less.cc: New test.
	* testsuite/19_diagnostics/error_condition/operators/not_equal.cc:
	Remove incorrect comment.
	* testsuite/19_diagnostics/error_condition/operators/three_way.cc: New
	test.