[llvm] [llvm-objdump] Support --mcpu=help/--mattr=help without -d (PR #165661)

[Ruoyu Qiu via llvm-commits]

cabbaken wrote:

> Okay, would you like me to merge this?

Yes, please.
Should I request commit access if I plan to do further development? 

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