NDK toolchain variables

Add variables for prebuilt toolchain directories and executables to help NDK build.

```javascript
//androidlib.env.find(options,done)

  {
   "ndkPath": "/opt/android-ndk-r9b",
   "sdkPath": "/opt/android-sdk/sdk",
   "arch": "arm",
   "archlib": "armeabi-v7a",
   "toolchain": "/opt/android-ndk-r9b/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86_64",
   "toolchain_version": "4.8",
   "toolchain_dir": "darwin-x86_64",
   "toolchain_llvm": "/opt/android-ndk-r9b/toolchains/llvm-3.3/prebuilt/darwin-x86_64",
   "toolchain_arch": "arm-linux-androideabi",
   "toolchain_ar": "/opt/android-ndk-r9b/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-ar",
   "toolchain_clang": "/opt/android-ndk-r9b/toolchains/llvm-3.3/prebuilt/darwin-x86_64/bin/clang++",
   "toolchain_platform_sdk": "/opt/android-ndk-r9b/platforms/android-19/arch-arm",
   "toolchain_libstdcpp": "/opt/android-ndk-r9b/sources/cxx-stl/gnu-libstdc++/4.8",
   "toolchain_libstdcpplib": "/opt/android-ndk-r9b/sources/cxx-stl/gnu-libstdc++/4.8/libs/armeabi-v7a",
   "jar": "/opt/android-sdk/sdk/platforms/android-19/android.jar"
}
```