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

[via llvm-commits]

bd1976bris 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.

@zeroomega is this repeatable? We haven't seen this on our windows builds.

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