[libc] Provide Arm Optimized Routines for the LLVM libc project.

This adds the Arm Optimized Routines (see
https://github.com/ARM-software/optimized-routines) source code under the
the LLVM license. The version of the code provided in this patch is v20.02
of the Arm Optimized Routines project.

This entire contribution is being committed as is even though it does
not currently fit the LLVM libc model and does not follow the LLVM
coding style. In the near future, implementations from this patch will be
moved over to their right place in the LLVM-libc tree. This will be done
over many small patches, all of which will go through the normal LLVM code
review process. See this libc-dev post for the plan:
http://lists.llvm.org/pipermail/libc-dev/2020-March/000044.html

Differential revision of the original upload: https://reviews.llvm.org/D75355