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

[Ruoyu Qiu via llvm-commits]

cabbaken wrote:

@jh7370 
Would it be necessary or more appropriate to create a separate PR to implement the object triple detection logic for `--mcpu=help`?

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