New x86 toolchain: x86_64-linux-android-4.8

This toolchain can generates x86, x32, and x86_64 (default) code.
Android build system already adds "-m32" to HOST_GLOBAL_CFLAGS
and HOST_GLOBAL_LDFLAGS for projects to compiled in x86.

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_64-4.8 --try-64
rm -rf $NDK/toolchains/x86_64-4.8/prebuilt/linux-x86_64/sysroot
cp -a $NDK/toolchains/x86_64-4.8/prebuilt/linux-x86_64/* \
  $AOSP/prebuilts/gcc/linux-x86_64/x86/x86_64-linux-android-4.8

Change-Id: Ibfb04cdb0ea9943f7b090c532df7832b6d0d35ef
244 files changed