Using __chk_fail from libstdc++

[Roland McGrath]

> On Fri, Jun 01, 2012 at 12:56:26PM -0700, Roland McGrath wrote:
> > > Nope, the above is a jmp __chk_fail at the assembly level, just the
> > > libstdc++ header doesn't provide that prototype in :: namespace.
> > 
> > OK, I see.  So that is cleaner from the C++ user perspective.
> > But it doesn't address Mike's concern about presuming the signature
> > will never change.
> 
> It better shouldn't, I don't see why it would be desirable.  ;)

I agree.  But I also agree with Mike's tendency toward pedanticism.
We should at least add some comments somewhere in libc saying that
__chk_fail is used by libstdc++ headers and must not change even though
it's not declared in any header.