Build instructions:

export AOSP=your-AOSP-path
export NDK=$AOSP/ndk
cd $NDK ; ./build/tools/dev-cleanup.sh
./build/tools/download-toolchain-sources.sh /tmp/ndk-$USER/src
./build/tools/gen-platforms.sh --minimal --dst-dir=$NDK --ndk-dir=$NDK --arch=x86_64
./build/tools/build-gcc.sh /tmp/ndk-$USER/src $NDK x86_64-4.9 --try-64
rm -rf $NDK/toolchains/x86_64-4.9/prebuilt/linux-x86_64/sysroot
cp -a $NDK/toolchains/x86_64-4.9/prebuilt/linux-x86_64/* \
  $AOSP/prebuilts/gcc/linux-x86_64/x86/x86_64-linux-android-4.9

Change-Id: I2efe2cbf64873551445765079662150fb3f93a64