Add Raspberry PI build (#11675)

* Add support for Raspberry PI (ARM) builds.

To build:

./configure

bazel build --sandbox_debug -c opt --copt=-march=armv6 --copt=-mfpu=vfp \
  --copt=-funsafe-math-optimizations --copt=-ftree-vectorize \
  --copt=-fomit-frame-pointer --cpu=armeabi \
  --crosstool_top=@local_config_arm_compiler//:toolchain \
  --verbose_failures --build_python_zip ...targets...

* Additional changes to get raspberry pi build working.

update platform.h and crosstool template for define RASPBERRY_PI.

* Add build_raspberry_pi.sh with some documentation.

* Cleanups for build_raspberry_pi.sh

* contrib/makefile/Makefile in PI mode: add -DRASPBERRY_PI.

* Recommend using aptitude instead of apt-get in build_raspberry_pi.sh.

* More comments to build_raspberry_pi.sh

* Switched to tar file for ARM tool chain

* More fixes to arm build scripts; move tools/arm_compiler -> third_party/toolchains/cpus/arm.

* Update comments in build_raspberry_pi: get ubuntu ports for your lsb codename.

* Hacked around cxx_builtin_include_directory problem in CROSSTOOL

* Added an extra system include

* Added Pi Two/Three NEON support, with fix for Eigen version

* Undo pete's Eigen change - it breaks OS X build.  Will be fixed later.