Using __chk_fail from libstdc++

[Roland McGrath]

I thought of that too but didn't think I cared.  We don't have an
existing header that seems appropriate.  Since it's an __ function,
it shouldn't really be declared in a "user" header.  So I guess it
would be some new header like <bits/fortify.h>.  But then libstdc++
would require a new-enough libc to have that header.  If this is
only going to be called from the library itself that doesn't seem
like a problem since it can just autoconf it.  But if Florian
intends to call it from libstdc++ header files (e.g. template code)
then it could be an issue.