[llvm-objcopy] --[de]compress-debug-sections: don't compress SHF_ALLOC...

[llvm-objcopy] --[de]compress-debug-sections: don't compress SHF_ALLOC sections, only decompress .debug sections

Simplify --[de]compress-debug-sections to make it easier to add custom section [de]compression.
Change the following two behaviors to match GNU objcopy.

* --compress-debug-sections compresses SHF_ALLOC sections while GNU
  doesn't.
* --decompress-debug-sections decompresses non-debug sections while GNU
  doesn't.

Pull Request: https://github.com/llvm/llvm-project/pull/84885