[RFC][ARM][CMSE] ARMv8-M Security Extensions - upstreaming clang/llvm support

Hi all:

I will soon be up-streaming our compiler support for Armv8-M Security Extensions. This email is to inform the community so that interested reviewers are aware when the patches go on Phabricator; and please do let me know if you want to be added as reviewer.

More details about the architecture can be obtained from [1]. The requirements on Development Tools is specified in [2]. Below I will mostly focus on the points necessary to understand llvm and clang implementation support patches. Armv8-M Security Extensions is in some context known as CMSE, and in the rest of this description I will refer to it therefore simply as CMSE.

  1. General Description