Merge LLVM Embedded Toolchain for Arm into arm-software/embedded

Merge https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm into arm-software/embedded

This also updates all the files with the toolchain's new name and directory structure. The main functional difference is that because the CMake scripts now live inside a fork of llvm-project, it is no longer necessary to checkout that repo. However the downstream patches remain for the moment as these have not yet been applied to the new repo, and will need to be manually applied before a build.

Documentation will likely need further updates, but these should be enough to start building.