Add CMake build instructions

* Add instructions for building for various platforms using CMake.

* Provide a CMake toolchain file for Arm GNU Toolchain.

Signed-off-by: Jakub Sujak <jakub.sujak@arm.com>

Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>
Approved-by: Jens Elofsson <jens.elofsson@arm.com>