[llvm] [llvm-objcopy] Handle SHF_ALLOC relocation sections in relocatable files (PR #227204)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This is Part 1 of #196381.  Part 2 is to preserve symbols with `st_shndx` in `[SHN_LOOS, SHN_HIOS]`

https://github.com/llvm/llvm-project/pull/227204