coreboot: make Ada support optional

Ada support keeps breaking due to gnat being in a somewhat poor state
in nixpkgs. As Ada support is only necessary for some Intel drivers
and these are generally not required for using coreboot in a VM, make
Ada support optional.

pkgs/development/tools/misc/coreboot-toolchain/default.nix

···
     , getopt
     , git
     , gnat11
+    , gcc
     , lib
     , perl
     , stdenvNoCC
     , zlib
+    , withAda ? true
     }:
 
     stdenvNoCC.mkDerivation rec {
···
       };
 
       nativeBuildInputs = [ bison curl git perl ];
-      buildInputs = [ flex gnat11 zlib ];
+      buildInputs = [ flex zlib (if withAda then gnat11 else gcc) ];
 
       enableParallelBuilding = true;
       dontConfigure = true;