[llvm-readelf] Render messages similar to that of `GNU binutils readelf` when...

[llvm-readelf] Render messages similar to that of `GNU binutils readelf` when no sections and/or no headers.

When there are no section headers section information printed by llvm-readelf is not useful and unnecessarily verbose.  When there are no program headers there's a similar verbose output shown when section mapping is requested. Simplifying the message shown in these cases to match the behavior of `GNU binuntils readelf`.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D130670