[Ada] Update documentation about non-symbolic traceback

[Marc Poulhiès via Gcc-patches]

This documents the limitation of addr2line with Position-Independent Code,
introduces the replacement tool gnatsymbolize and adjusts obsolete stuff.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * doc/gnat_ugn/gnat_and_program_execution.rst
        (Non-Symbolic Traceback): Update section.
        * gnat_rm.texi, gnat_ugn.texi, gnat-style.texi: Regenerate.

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