[llvm] [llvm-objcopy] Allow -p on COFF targets (PR #171237)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

It looks like in adopting the ELF test, you might have lost the coverage for "already preserves the timestamp in the COFF header", which I believe was there before by doing a direct `cmp` of the before and after file. Is that correct?

https://github.com/llvm/llvm-project/pull/171237