libstdc++: Add C++11 member functions for ios::failure in old ABI

libstdc++: Add C++11 member functions for ios::failure in old ABI

The new constructors that C++11 added to std::ios_base::failure were
missing for the old ABI. This adds them, but just ignores the
std::error_code argument (because there's nowhere to store it).

This also adds a code() member, which should be provided by the
std::system_error base class, but that base class isn't present in the
old ABI.

This allows the old ios::failure to be used in code that expects the new
API, although with reduced functionality.

libstdc++-v3/ChangeLog:

	* include/bits/ios_base.h (ios_base::failure): Add constructors
	takeing error_code argument. Add code() member function.
	* testsuite/27_io/ios_base/failure/cxx11.cc: Allow test to
	run for the old ABI but do not check for derivation from
	std::system_error.
	* testsuite/27_io/ios_base/failure/error_code.cc: New test.