This is a working in progress patch, and it needs clean up and refactor. It is mainly for understanding what are needed to build windows runtime when the LLVM is being cross compiled for a different architecture. And to discuss the direction of refactoring the cmake configurations for the clang bootstrap build as well as LLVM runtimes build.
The use case is to include the Windows runtime build when cross compiling LLVM for an arch different from the host (e.g. building aarch64 on an x64 machine). This will be needed in the future when cross compile LLVM Windows toolchain for aarch64 on an x64 machine.