[llvm] [llvm-readobj,ELF] Support --decompress/-z (PR #82594)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Thanks!
> 
> For an unsupported case I thought you might have a non-zlib/zstd case,
> 
> ```
> ...
>     Flags: [SHF_COMPRESSED]
>     Content: 03.....
> ```

Thanks. Added

https://github.com/llvm/llvm-project/pull/82594