android: add ndkVer to resolve ndk ambiguity

It wasn’t exactly clear which NDK you were using previously. This adds
an attribute to system that handles what version of the NDK we should
use when building things.

/cc @Ericson2314

pkgs/stdenv/cross/default.nix

@@ -42,7 +42,7 @@ in lib.init bootStages ++ [
       cc = if crossSystem.useiOSPrebuilt or false
              then buildPackages.darwin.iosSdkPkgs.clang
            else if crossSystem.useAndroidPrebuilt
-             then buildPackages.androidenv.androidndkPkgs.gcc
+             then buildPackages.androidenv."androidndkPkgs_${crossSystem.ndkVer}".gcc
            else buildPackages.gcc;
     };
   })