[llvm] cmake: Derive CMake system name from a triple via new mechanism (PR #208773)

[Douglas Yung via llvm-commits]

dyung wrote:

If we are not going to revert this change, can we at least get a fix for the python path issue on Windows? This Windows bot has been failing since this change was landed:

https://lab.llvm.org/buildbot/#/builders/46/builds/38942

https://github.com/llvm/llvm-project/pull/208773