androidndk: Add makeStandaloneToolchain support

Example:
```
let
  toolchain = (androidenv.androidndk.makeStandaloneToolchain 24 "arm64");
in
...
```

pkgs/development/mobile/androidenv/androidndk.nix

@@ -1,12 +1,18 @@
 { stdenv, fetchurl, zlib, ncurses5, unzip, lib, makeWrapper
 , coreutils, file, findutils, gawk, gnugrep, gnused, jdk, which
-, platformTools, python3, libcxx, version, sha256, bash
+, platformTools, python3, libcxx, version, sha256, bash, runCommand
 , fullNDK ? false # set to true if you want other parts of the NDK
                   # that is not used by Nixpkgs like sources,
                   # examples, docs, or LLVM toolchains
 }:
 
-stdenv.mkDerivation rec {
+let
+  makeStandaloneToolchain = api: arch: let
+      full_ndk = (ndk true);
+    in runCommand "makeStandaloneToolchain-${version}" {} ''
+      ${full_ndk}/libexec/${full_ndk.name}/build/tools/make_standalone_toolchain.py --api ${toString api} --arch ${arch} --install-dir $out
+    '';
+  ndk = fullNDK: stdenv.mkDerivation rec {
     name = "android-ndk-r${version}";
     inherit version;
 
@@ -47,7 +53,7 @@ stdenv.mkDerivation rec {
           ''
             patch -p1 \
               --no-backup-if-mismatch \
-            -d $out/libexec/${name} < ${ ./. + "/make_standalone_toolchain.py_${version}.patch" }
+              -d $out/libexec/${name} < ${ ./. + "/make_standalone_toolchain.py_" + "${version}" + ".patch" }
 
             sed -i 's,#!/usr/bin/env python,#!${python3}/bin/python,g' ${pkg_path}/build/tools/make_standalone_toolchain.py
             sed -i 's,#!/bin/bash,#!${bash}/bin/bash,g' ${pkg_path}/build/tools/make_standalone_toolchain.py
@@ -97,4 +103,8 @@ stdenv.mkDerivation rec {
       hydraPlatforms = [];
       license = stdenv.lib.licenses.asl20;
     };
-}
+  };
+  passthru = {
+    inherit makeStandaloneToolchain;
+  };
+in lib.extendDerivation true passthru (ndk fullNDK)