gcc-snapshot: mark as broken

It is older than gcc7 and fails to compile due to ucontext_t changes in glibc 2.26.

pkgs/development/compilers/gcc/snapshot/default.nix

···
       stdenv.lib.platforms.linux ++
       stdenv.lib.platforms.freebsd ++
       optionals (langAda == false) stdenv.lib.platforms.darwin;
+
+    broken = true;
   };
 }