[llvm] [llvm-otool] Fix -arch flag for universal binary slice selection (PR #184810)

[Daniel Rodríguez Troitiño via llvm-commits]

https://github.com/drodriguez approved this pull request.

Sorry for the delay. I did not realize I was the only reviewer.

The changes makes sense to me and from what I can see follow the patterns that I can find in the code to deal with the `otool` / `llvm-objdump` split.

https://github.com/llvm/llvm-project/pull/184810