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

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

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.

Reviewed By: jhenderson, phosek

Differential Revision: https://reviews.llvm.org/D134385