[llvm] [Object][Archive] Recompute headers and symbol map when switching from COFF to GNU64 (PR #160606)

[Nico Weber via llvm-commits]

https://github.com/nico approved this pull request.

Thanks!

If link.exe can handle GNU files, maybe we should default to GNU in LibDriver.cpp unless we're targeting ARM64EC?

But I'm happy with anything that unbreaks things.

https://github.com/llvm/llvm-project/pull/160606