[llvm-objcopy] --compress-debug-sections: remove tail padding for ELFCLASS32

[MaskRay]

Summary

For an ELFCLASS32 object, a compressed section due to --compress-debug-sections={zlib,zstd} has a
tail padding of 12 zero bytes. zlib happily allows this while zstd is rigid and
reports error: '{{.*}}': failed to decompress section '.debug_foo': Src size is incorrect.

Cole Kissane reported the problem to me.

[phosek]

LGTM

[jhenderson]

LGTM too!

[jhenderson]

llvm/lib/ObjCopy/ELF/ELFObject.cpp

538–540

Actually one question: this seems to be assuming that BE and LE Elf_Chdr_Impl are the same size. I assume that's intentional, and it's fine if it is, but perhaps it deserves a static_assert or similar to protect us in case something odd changes in the future?

[MaskRay]

llvm/lib/ObjCopy/ELF/ELFObject.cpp

538–540

Thanks for the quick review. ELF has ELFCLASS32 and ELFCLASS64 distinction and endianness does not affect the structures. LLVMObject defines Elf_Chdr_Impl to provide ergonomic access for cross-endianness situations but I feel static_assert is rather unnecessary here.