[Ada] Document the SuSE kernel bug

[Pierre-Marie de Rodat via Gcc-patches]

We found a SuSE kernel bug that affects 32-bit debugging on 64-bit
machines.  We agreed to document the problem in gnat_ugn.

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

gcc/ada/

        * doc/gnat_ugn/platform_specific_information.rst: Document the
        SuSE kernel bug.
        * gnat_ugn.texi: Regenerate.

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