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

[via llvm-commits]

zeroomega wrote:

We also see a weird issue on Windows is that during clang test step, lld timed out:
`Linking CXX executable unittests\Transforms\Scalar\ScalarTests.exe`  took 36 minutes and caused the bot timed out. 
Your change is the only on the blamelist. 

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