[llvm] [llvm-objdump] Add triple support to `mcpu=help` (PR #165661)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

I think you could do with additional test cases to cover/better illustrate certain behaviours, as described in my previous review. Specifically:
- No -d, --triple, or input file (this should be a failure, but it might be clearer to explicitly report a dedicated error if --mcpu=help has been requested saying something about being unable to determine the triple to produce help for, rather than relying on "a.out not found" or similar error).
- --mcpu=help + some other dump option e.g. dump file headers, to show that other non-disassembly stuff is still done. This covers the inverse of point 4b from my previous review.
Additionally, tests should be amended to ensure the help is only printed once and not two or more times.

What do you think about extending this PR to cover `--mattr=help` too, rather than waiting for a separate PR?

https://github.com/llvm/llvm-project/pull/165661