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

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.