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.


Tested powerpc64le-linux, committed to master.