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.
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