Add C11 noreturn

If it's unavailable, it's defined to __attribute__((noreturn)).

Unfortunately, it cannot be used for ObjC methods, as noreturn is part
of the return type while __attribute__((noreturn)) needs to be at the
end for an ObjC method. To make matters worse, even GCC versions that
accept noreturn don't allow it for an ObjC method. Thus, the only thing
that can be done is to always use __attribute__((noreturn)) for ObjC
methods using the OF_METHOD_NORETURN define.