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

[Ruoyu Qiu via llvm-commits]

cabbaken wrote:

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

Will add `--mattr=help` later in this PR.

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