gnat{6,9,10,11}: don't build libada when building a cross compiler

According to https://wiki.osdev.org/GNAT_Cross-Compiler building
libada is not possible when building a cross compiler. Unfortunately I
haven't been able to determine if this is upstream's position as well,
but sure enough disabling libada lets us build a GNAT cross compiler.

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -170,8 +170,11 @@ let
       "--enable-cloog-backend=isl"
     ]
 
-    # Ada options
-    ++ lib.optional langAda "--enable-libada"
+    # Ada options, gcc can't build the runtime library for a cross compiler
+    ++ lib.optional langAda
+      (if hostPlatform == targetPlatform
+       then "--enable-libada"
+       else "--disable-libada")
 
     # Java options
     ++ lib.optionals langJava [