[lld] [ELF] Change Ctx::target to unique_ptr (PR #111260)

[Paul Kirth via llvm-commits]

ilovepi wrote:

I just posted a revert in: https://github.com/llvm/llvm-project/pull/111449

Since this seems to be breaking the windows build bots. We can always reland later with a fixed up version of the patch.

https://github.com/llvm/llvm-project/pull/111260