[llvm] [ARM] Use TargetMachine over Subtarget in ARMAsmPrinter (PR #166329)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

This looks good to me. All the changes relate to things that don't vary per function like the object file format.

Will be good to leave some time for reviewers in the US time zone to comment.

https://github.com/llvm/llvm-project/pull/166329