[llvm] [llvm][docs] Refresh Arm builtins cross compile guide (PR #150966)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Great I'll land this then.

If this guide works with just the compiler-rt sources, nothing else, then I might leave it as is actually. We don't check them often but in theory standalone builds are as legit as the runtimes build is. I'll see if it works.

https://github.com/llvm/llvm-project/pull/150966