RFC: Add GNU_PROPERTY_1_GLIBC_2_NEEDED

[Florian Weimer]

* H. J. Lu:

> How do we deal with
>
> Somes binaries compiled with the new language features, like C2X
> printf specifiers, only generate correct results with the new glibc
> binary.  Since we don't add new glibc versions to the printf function
> family, they generate incorrect results at run-time with the older
> glibc binaries.

We decided not to use symbol versions for that.  Likewise for support
for new flags in e.g. dlopen.  We also do not od this for system calls
and system call flags.

If an application uses the new printf specifiers, it can probe for
support at run time and register its own implementation if necessary.
This is a bit complicated, but it's how applications are expected to
handle new flags in older interfaces.

Thanks,
Florian