[llvm] [llvm-objdump] Default to --mattr=+special for AVR (PR #174731)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The new function `getFeatureSetForEFlag` doesn't have much coverage. Can you improve the coverage? Can you add a note why something like `+all` cannot be used for AVR?

https://github.com/llvm/llvm-project/pull/174731