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

[Petr Hosek via llvm-commits]

petrhosek wrote:

In addition to introducing a hard dependency on Python, this change also ties LLVM and CMake closer together without a clear contract. To give a concrete example, say the next release of CMake adds platform support for UEFI (this is a very likely scenario since that's something we're working on); would we immediately update `llvm/include/llvm/TargetParser/TripleName.def` and break anyone using older CMake versions, or would we follow the LLVM's `cmake_minimum_required` which means the new mechanism wouldn't be helpful until LLVM bumps the minimum CMake version? I believe this deserves an RFC and the policy needs to be properly documented.

> > That's not necessarily a problem, I guess, but we'd want to update the system requirements, and fix CMake to give a god error message if it's missing.
> 
> The top level build already does `find_package(Python3 REQUIRED)`, python has been a hard requirement for a very long time?

Python is a hard dependency for running tests but shouldn't be required to build LLVM, although the build system and our documentation doesn't accurately reflect that.

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