glibc SFrame support for newer versions

[Florian Weimer]

* Indu Bhagat:

> Aside: When time comes to set --enable-sframe by default though, the
> system call to register SFrame data for (dlopen'd) shared libraries
> should ideally have landed too.

I think userspace backtracing can be independently useful, once the
format is ready.  Today, we use DWARF in userspace, and it does not
matter that the kernel does not want to deal with it.

Thanks,
Florian