[llvm] [llvm][docs] Update CMake commands for cross compiling Arm builtins (PR #151544)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I looked at whether it should use the runtimes build and maybe but it's easier to not have to explain any of that here. It's still valid, as far as I know, to configure using `compiler-rt/`.

https://github.com/llvm/llvm-project/pull/151544