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.
* Switch to calling `putchar_unlocked()` instead of `putchar()` for
actual printing. We're already locking stdout with `flockfile()`, so
there's no need for the redundant lock that `putchar()` uses.
* Add an explicit lock to the output stream in `dump()`. This means the
entire dump is printed with the lock held, which will prevent the
output of `dump()` from mixing with prints on other threads.
* Use `_debugPrint_unlocked()` instead of `debugPrint()` in
`_adHocPrint()`. The output stream is already locked while this
function is executing. Rename the function to `_adHocPrint_unlocked()`
to make this explicit.
* Use `targetStream.write()` and `_print_unlocked()` instead of
`print()` in `_dumpObject()`. This removes the redundant locking, and
also eliminates the creation of intermediate strings. Rename the
function to `_dumpObject_unlocked()` to make this explicit.
* Use `targetStream.write()`, `_print_unlocked()`, and
`_debugPrint_unlocked()` in `_dumpSuperclass()`. This removes the
redundant locking, and also eliminates the creation of intermediate
strings. Rename the function to `_dumpSuperclass_unlocked()` to make
this explicit.
* Use `_debugPrint_unlocked()` instead of `debugPrint()` in
`String.init(reflecting:)`. This shouldn't really make much of a
difference but it matches the usage of `_print_unlocked()` in
`String.init(_:)`.
The net result is that all printing is still covered under locks like
before, but stdout is never recursively locked. This should result in
slightly faster printing. In addition, `dump()` is now covered under a
single lock so it can't mix its output with prints from other threads.
Set up a separate libSwiftStubs.a archive for C++ stub functionality that's needed by the standard library but not part of the core runtime interface. Seed it with the Stubs.cpp and LibcShims.cpp files, which consist only of stubs, though a few stubs are still strewn across the runtime code base.
