Add a changelog for the current release.

Change-Id: Id9def30fbcf712e202e3980e50730ffaebefa2bc

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+Changelog for NDK Build 2490520
+===============================
+
+TODO(danalbert): Link to bug tracker.
+
+Clang
+-----
+
+ * **PSA: Everyone should be switching to Clang.**
+ * Clang has been updated to 3.8svn (r243773, build 2481030).
+     * Note that this is now a nearly pure upstream clang.
+     * Also note that Clang packaged in the Windows 64 NDK is actually 32-bit.
+ * Support for emulated TLS.
+     * `__thread` is now supported by the compiler by emulating ELF TLS with
+       pthread thread-specific data.
+     * C++11 `thread_local` will work in some cases, but will not work for data
+       with non-trivial destructors except when running on Marshmallow
+       (android-23) or newer because those cases require support from libc.
+     * Does not yet work with Aarch64 when TLS variables are accessed from a
+       shared library.
+
+GCC
+---
+
+ * **GCC in the NDK is now deprecated.**
+     * Time to start using Clang if you haven’t already. If you have problems
+       with Clang, please file bugs!
+     * The NDK will not be upgrading to 5.x, nor will we be accepting
+       non-critical backports.
+     * Maintenance for miscompiles and internal compiler errors in 4.9 will be
+       handled on a case by case basis.
+ * GCC 4.8 has been removed. All targets now use GCC 4.9.
+ * Synchronized with `google/gcc-4_9` to r224707 (from r214835).
+
+NDK
+---
+
+ * The samples are no longer included in the NDK. They are instead available on
+   [GitHub].
+ * The documentation is no longer included in the NDK. It is instead available
+   on the [Android Developer website].
+ * The layout of the NDK has changed quite a bit. This was to facilitate
+   splitting the many components into their own packages.
+     * This is likely to cause some disruption during the transition from r10
+       for anyone not using `ndk-build` or `make-standalone-toolchain.sh`, as
+       the compilers have moved.
+ * Make ARM standalone toolchains default to arm7.
+     * The old behavior can be restored by passing
+       `-target armv5te-linux-androideabi`.
+ * Use `-isystem` for platform includes.
+     * Warnings caused by bionic will no longer break app builds.
+ * Fixed segfault when throwing exceptions via gabi++ (see
+   http://b.android.com/179410).
+ * Change libc++’s inline namespace to `std::__ndk1` to prevent ODR issues with
+   platform libc++.
+ * Support for mips64r2 has been partially dropped. The rest will be dropped in
+   the future.
+ * All libc++ libraries are now built with libc++abi.
+ * Bump default `APP_PLATFORM` to Gingerbread.
+     * Expect support for Froyo and older to be dropped in a future release.
+ * Updated gabi++ `_Unwind_Exception` struct for 64 bits.
+ * cpufeatures: Detect SSE4.1 and SSE4.2 as well.
+ * cpufeatures: Detect cpu features on x86\_64 as well.
+ * Update libc++abi to upstream r231075.
+ * Added native tracing API to android-23.
+ * Added native multinetwork API to android-23.
+ * `byteswap.h`, `endian.h`, `sys/procfs.h`, `sys/ucontext.h`, `sys/user.h`, and
+   `uchar.h` have all been updated from ToT Bionic.
+ * `sys/cdefs.h` has been synchronized across all API levels.
+ * Support for `_WCHAR_IS_8BIT` has been removed.
+ * Fixed `fegetenv` and `fesetenv` for arm
+ * Fix end pointer size/alignment of `crtend_*` for mips64 and x86\_64
+ * Removed sed.
+ * Removed mclinker.
+ * Removed Perl.
+
+Binutils
+--------
+
+ * Unified binutils source between Android and ChromiumOS.
+ * For full details see https://android-review.googlesource.com/#/c/182865/.
+ * Gold for aarch64 should now be much more reliable. Use `-fuse-ld=gold` at
+   link time to use gold instead of bfd. The default will likely switch in the
+   next release.
+ * Good linking time improvement for huge binaries for Gold ARM backend (up to
+   50% linking time reduction for debuggable Chrome Browser).
+ * New option: `--pic-veneer`.
+
+GDB
+---
+
+TODO(jmgao)
+
+Yasm
+----
+
+ * Yasm has been updated to version 1.3.0.
+
+Known Issues
+------------
+
+ * This is not intended to be a comprehensive list of all outstanding bugs.
+
+ * x86 ASAN does not currently work. See discussion on
+   https://android-review.googlesource.com/#/c/186276/
+ * The combination of Clang, x86, `stlport_static`, and optimization levels
+   higher than `-O0` causes test failures with `dynamic_cast`. See
+   https://android-review.googlesource.com/#/c/185920
+ * Exception handling will often fail when using `c++_shared` on ARM32. The root
+   cause is incompatibility between the LLVM unwinder used by libc++abi for
+   ARM32 and libgcc. This is not a regression from r10e.
+
+What's Next?
+------------
+
+A significant proportion of the changes that went into this release were
+organizational changes to the build and to the package layout. The reason for
+this is that we will soon be integrating the NDK into the Android SDK manager as
+several distinct components. The motivations for this change are:
+
+ * No wasted bandwidth or disk space on unneeded pieces. Do you really need 14
+   compilers? With the modular NDK you can download just the one Clang, or a
+   single set of GCCs.
+ * Easier updating. Just launch the SDK manager and install the updates rather
+   than downloading an entire new NDK.
+ * Quicker updates. In the future we’ll be able to release just a portion of the
+   NDK on a quicker schedule. No more waiting for the next big release for a
+   handful of bug fixes or a compiler update.
+
+The integration with the SDK manager should happen Soon™.
+
+[GitHub]: https://github.com/googlesamples/android-ndk
+[Android Developer website]: http://developer.android.com/ndk/index.html