Bug#861734: gcc-7 fails to build gnattools on armel

[Nicolas Boulenguez]

Package: src:gcc-7
Followup-For: Bug #861734

Please build the native gnat on armel by reverting
svn://anonscm.debian.org/gcccvs/branches/sid/gcc-7@9434 
6ca36cf4-e1d1-0310-8c6f-e303bb2178ca
Since then, the link step for the tools has changed a lot, and a
recent build log would be more helpful. Moreover, --as-needed -z,defs
have been added to libgnat linker options, and should help locating
the problem.