…eabi-v7a, build icu 55.1

Changed default NDK version from 4.8 to 4.9

Seems like `RELEASE.TXT` is gone, so `arch` is now used to
determine if we're running on 64bit or not. I'm unsure how
to properly check for this on Windows, though.

Android NDK r13 appears to have removed a level of nesting
within LLVM libc++. The "libcxx" subdirectory no longer exists.

Swift is expecting this libraries to be named `icuucswift` for Android to avoid name collision with Android's `icuuc`.

This should be temporal, a more appropriate solution (if any) should be applied to solve this lib conflict.

These patches _should_ be able to be applied using `patch`, they
automate the changeset currently described at apple/swift/docs/Android.md