PR libstdc++/60555 std::system_category() should recognise POSIX errno values

PR libstdc++/60555 std::system_category() should recognise POSIX errno values

	PR libstdc++/60555
	* src/c++11/system_error.cc
	(system_error_category::default_error_condition): New override to
	check for POSIX errno values.
	* testsuite/19_diagnostics/error_category/generic_category.cc: New
	* testsuite/19_diagnostics/error_category/system_category.cc: New
	test.

From-SVN: r263210