[llvm] [llvm-objcopy] Add --compress-sections (PR #85036)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Sorry, I was waiting for some sort of traction on the RFC (of which there hasn't really been any from what I can tell).
> 
> I'll go through the PR as I have nothing specifically against the option, but I think there needs to be some clarity that there is more widespread appeal of this option.

Thanks. This is to complement [[[RFC] Compress arbitrary sections with ld.lld –compress-sections](https://discourse.llvm.org/t/rfc-compress-arbitrary-sections-with-ld-lld-compress-sections/71674), so that `ld.lld --compress-sections=...` compressed sections can be decompressed. My previous refactoring and simplification is to decrease the complexity.

[[RFC] Compressed SHT_SYMTAB/SHT_STRTAB for ELF](https://discourse.llvm.org/t/rfc-compressed-sht-symtab-sht-strtab-for-elf/77608) is a related proposal that doesn't get much traction yet. While I agree we should be more careful about whether we should go that direction (supporting `SHF_COMPRESSED SHT_STRTAB`), I think its acceptance is less related to this patch.

https://github.com/llvm/llvm-project/pull/85036