swift234

Port to Android

This adds an Android target for the stdlib. It is also the first
example of cross-compiling outside of Darwin.

Mailing list discussions:

1. https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151207/000171.html
2. https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000492.html

The Android variant of Swift may be built using the following `build-script`
invocation:

```
$ utils/build-script \
  -R \                                           # Build in ReleaseAssert mode.
  --android \                                    # Build for Android.
  --android-ndk ~/android-ndk-r10e \             # Path to an Android NDK.
  --android-ndk-version 21 \
  --android-icu-uc ~/libicu-android/armeabi-v7a/libicuuc.so \
  --android-icu-uc-include ~/libicu-android/armeabi-v7a/icu/source/common \
  --android-icu-i18n ~/libicu-android/armeabi-v7a/libicui18n.so \
  --android-icu-i18n-include ~/libicu-android/armeabi-v7a/icu/source/i18n/
```

Android builds have the following dependencies, as can be seen in
the build script invocation:

1. An Android NDK of version 21 or greater, available to download
   here: http://developer.android.com/ndk/downloads/index.html.
2. A libicu compatible with android-armv7.