[PATCH] S390: Remove backchain-based fallback and use generic backtrace.c.

[Florian Weimer]

* Stefan Liebler:

> After recent discussions:
> - "[PATCH] s390: Remove backchain-based fallback from backtrace"
>   https://www.sourceware.org/ml/libc-alpha/2020-02/msg00287.html
>
> - "Re: [PATCH 07/11] s390: Implement backtrace on top of <unwind-link.h>"
>   https://www.sourceware.org/ml/libc-alpha/2020-02/msg00637.html
>
> We've checked and decided to remove the backchain:
> We don't know of any environments without libgcc. Thus the backchain
> unwinder is not used. If somebody builds with -mbackchain and without
> fasynchronous-unwind-tables and has libgcc installed, then the
> libgcc unwinder is called but not the backchain-based fallback.
>
> This step allows to get rid of the s390x specific backtrace.c files at all.
> Furthermore the now used debug/backtrace.c version has some more
> advantages:
> - Free all resources if necessary. (libc_freeres_fn)
> - Remove NULL address above _start.
> - Check whether we make any progress while getting addresses.

Looks good to me.  Thanks for doing this.

Florian